
<PAGE>

                                                                     Exhibit 5.1


                                 April 11, 2001



Credit Suisse First Boston Corporation
Banc of America Securities LLC
SunTrust Equitable Securities Corporation
Scotia Capital (USA) Inc.
BNY Capital Markets, Inc.
c/o Credit Suisse First Boston Corporation
11 Madison Avenue
New York, New York 10172


Ladies and Gentlemen:

          We have acted as special counsel to DaVita Inc., a Delaware
corporation (the "Issuer"), and the subsidiary guarantors identified on Schedule
A attached hereto (the "Guarantors"), in connection with the Operative Documents
(as defined below), including that certain Purchase Agreement (the "Purchase
Agreement"), dated April 6, 2001, by and among the Issuer, the Guarantors and
the Initial Purchasers named therein (collectively, the "Purchasers"), relating
to the sale by the Issuer of $225,000,000 aggregate principal amount of the
Issuer's 9  1/4% Senior Subordinated Notes due 2011 (the "Notes"). This opinion
is delivered to you in compliance with Section 9(e) of the Purchase Agreement.
Capitalized terms used in this opinion which are not defined herein shall have
the meanings given to them in the Purchase Agreement.

          In connection with this opinion, we have examined executed originals,
counterparts or copies identified to our satisfaction as being true copies of
such certificates, records, documents or other instruments as we have deemed
necessary or appropriate to enable us to render the opinions expressed below.
These certificates, records, documents and other instruments included the
following:

          (a)  The Certificate of Incorporation of the Issuer, as amended to
date, and comparable organizational documents of the Guarantors;

          (b)  The Bylaws of the Issuer, as amended to date, and comparable
organizational documents of the Guarantors;
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Credit Suisse First Boston Corporation
Banc of America Securities LLC
SunTrust Equitable Securities Corporation
Scotia Capital (USA) Inc.
BNY Capital Markets, Inc.
April 11, 2001
Page 2

          (c)  The records of certain proceedings and actions taken by the
Issuer and the Guarantors relating to the transactions contemplated by the
Purchase Agreement, which have been certified to us as constituting all of the
proceedings and actions relating thereto;

          (d)  The Purchase Agreement;

          (e)  The Indenture, the form of Series A Notes and the form of Series
B Notes attached to the Indenture and the form of Subsidiary Guarantees attached
to the Indenture; and

          (f)  The Registration Rights Agreement.

          The documents referenced in items (d) through (f) above are
collectively referred to herein as the "Operative Documents."

          We have been furnished with, and with your consent have relied upon,
certificates of officers of the Issuer and the Guarantors with respect to
certain factual matters.  In addition, we have obtained and relied upon such
certificates and assurances from public officials as we have deemed necessary.
In all of our examinations, we have assumed the authenticity of all documents
submitted to us as original or certified documents, the genuineness of all
signatures on original or certified documents, the conformity to original
documents of all documents submitted to us as copies thereof and the correctness
and accuracy of all facts not independently established by us set forth in all
certificates and reports identified in this opinion.

          We have investigated such questions of law for the purpose of
rendering this opinion as we have deemed necessary.  We are attorneys duly
admitted and qualified to practice only in the State of California, and we are
opining herein as to the effect on the subject transactions of only United
States federal law, the General Corporation Law of the State of Delaware and the
laws of the State of California.  We are not opining on, and assume no
responsibility as to the applicability to, or the effect on any of the matters
covered herein of,
<PAGE>

Credit Suisse First Boston Corporation
Banc of America Securities LLC
SunTrust Equitable Securities Corporation
Scotia Capital (USA) Inc.
BNY Capital Markets, Inc.
April 11, 2001
Page 3

the laws of any other jurisdiction. We note that the Operative Documents are, by
their terms, governed by the laws of the State of New York. We are not admitted
in New York and we are not opining as to the laws of the State of New York. With
your consent, we are delivering the opinion set forth herein assuming that the
Operative Documents are governed by the laws of the State of California (without
reference to the choice of law principles thereunder). Without independent check
or verification, we are not aware of any exceptions to enforceability of the
Operative Documents under the laws governing the same that would be required to
be stated in our opinion if we were opining as to the laws governing such
documents but are not so stated in our opinion due to the fact that our opinion
is given with respect to California law. We are not expressing any opinion as to
the effect of compliance by the Purchasers with any state or federal laws or
regulations applicable to the transactions because of the nature of any of their
businesses. We express no opinion as to state securities or "blue sky" laws. In
addition, we are not expressing any opinion as to the effect of compliance with
applicable law by any party to the Operative Documents other than the Issuer and
the Guarantors.

          To the extent that the obligations of the Issuer or the Guarantors may
be dependent upon such matters, we have assumed for purposes of this opinion,
other than with respect to the Issuer and the Guarantors, that each party to the
agreements and contracts referred to herein is duly organized, validly existing
and in good standing under the laws of its jurisdiction of organization; that
each such other party has the requisite corporate or other organizational power
and authority to perform its obligations under such agreements and contracts, as
applicable; and that such agreements and contracts have been duly authorized,
executed and delivered by, and each of them constitutes the legally valid and
binding obligation of, such other parties, as applicable, enforceable against
such other parties in accordance with their respective terms.

          On the basis of the foregoing, and in reliance thereon, and subject to
the limitations, qualifications and exceptions set forth below, we are of the
opinion that:

          (1)  The Issuer has been duly organized, is validly existing and is in
good standing under the laws of the State of Delaware and has the corporate
power and authority to
<PAGE>

Credit Suisse First Boston Corporation
Banc of America Securities LLC
SunTrust Equitable Securities Corporation
Scotia Capital (USA) Inc.
BNY Capital Markets, Inc.
April 11, 2001
Page 4

carry on its business as described in the Offering Circular and to own, lease
and operate its properties;

          (2)  The Series A Notes have been duly authorized and, when executed
and authenticated in accordance with the provisions of the Indenture and
delivered to and paid for by the Purchasers in accordance with the terms of the
Purchase Agreement, will be entitled to the benefits of the Indenture and will
be valid and binding obligations of the Issuer, enforceable in accordance with
their terms.

          (3)  The Subsidiary Guarantees have been duly authorized and, when the
Series A Notes are executed and authenticated in accordance with the provisions
of the Indenture and delivered to and paid for by the Purchasers in accordance
with the terms of the Purchase Agreement, the Subsidiary Guarantees endorsed
thereon will be valid and binding obligations of the Guarantors, enforceable in
accordance with their terms.

          (4)  The Indenture has been duly authorized, executed and delivered by
the Issuer and each Guarantor and is a valid and binding agreement of the Issuer
and each Guarantor, enforceable against the Issuer and each Guarantor in
accordance with its terms.

          (5)  The Purchase Agreement has been duly authorized, executed and
delivered by the Issuer and the Guarantors.

          (6)  The Registration Rights Agreement has been duly authorized,
executed and delivered by the Issuer and the Guarantors and is a valid and
binding agreement of the Issuer and each Guarantor, enforceable against the
Issuer and each Guarantor in accordance with its terms.

          (7)  The Series B Notes have been duly authorized and, when executed
and authenticated in accordance with the provisions of the Indenture and
delivered in exchange for Series A Notes in accordance with the provisions of
the Indenture and the Exchange Offer, will be entitled to the benefits of the
Indenture and will be valid and binding obligations of the Issuer, enforceable
in accordance with their terms.
<PAGE>

Credit Suisse First Boston Corporation
Banc of America Securities LLC
SunTrust Equitable Securities Corporation
Scotia Capital (USA) Inc.
BNY Capital Markets, Inc.
April 11, 2001
Page 5

          (8)  When the Series B Notes have been executed and authenticated in
accordance with the provisions of the Indenture and delivered in exchange for
Series A Notes in accordance with the provisions of the Indenture and Exchange
Offer, the Subsidiary Guarantees endorsed thereon will be valid and binding
obligations of the Guarantors, enforceable in accordance with their terms.

          (9)  The statements under the captions "Certain Relationships and
Related Transaction," "Description of Debt" "Description of Notes," and "Federal
Income Tax Consequences to Non-U.S. Holders" in the Offering Circular, insofar
as such statements constitute a summary of legal matters, documents or
proceedings referred to therein, fairly present in all material respects such
legal matters, documents or proceedings.

          (10) Provided the Notes are sold in the manner contemplated by the
Purchase Agreement, the execution, delivery and performance of the Purchase
Agreement and the other Operative Documents by the Issuer and each of the
Guarantors, compliance by the Issuer and each of the Guarantors with all the
provisions thereof and the consummation of the transactions contemplated thereby
will not (i) require any consent, approval, authorization or other order of any
California or federal court, regulatory body, administrative agency or other
governmental body (except  such as have been obtained or as may be required
under the securities or Blue Sky laws of the various states and, with respect to
the Registration Rights Agreement, the Securities Act and the Trust Indenture
Act) or (ii) conflict with or constitute a breach of any of the terms or
provisions of, or a default under, (A) the charter or by-laws or other
organizational documents of the Issuer or any of its subsidiaries or (B) the
Credit Facilities, after receipt of any required consents, which consents have
been obtained; the Indenture, dated as of June 12, 1996, between Renal Treatment
Centers, Inc. and PNC Bank, National Association, including the first and second
supplemental indentures thereto; the Guaranty, dated March 31, 1998, made by the
Issuer in favor of PNC Bank, National Association; or the Indenture, dated as of
November 18, 1998, between the Issuer  and United States Trust Company of New
York except, with respect to clause (ii)(B), for any breach or default which
would not, singly or in the aggregate, have a Material Adverse Effect; or (ii)
violate or conflict with those laws, rules and regulations which, in our
experience, are normally applicable to transactions of the type contemplated by
the Operative Documents.
<PAGE>

Credit Suisse First Boston Corporation
Banc of America Securities LLC
SunTrust Equitable Securities Corporation
Scotia Capital (USA) Inc.
BNY Capital Markets, Inc.
April 11, 2001
Page 6

          (11) The Issuer is not and, upon the offering and sale of the Series A
Notes and the application of the net proceeds thereof as described in the
Offering Circular, will not be an "investment company" or a company "controlled"
by an "investment company" as such term is defined in the Investment Company Act
of 1940, as amended.

          (12) The Indenture complies as to form in all material respects with
the requirements of the Trust Indenture Act of 1939, as amended (the "TIA"), and
the rules and regulations of the Commission applicable to an indenture which is
qualified thereunder.  It is not necessary in connection the offer, sale and
delivery of the Series A Notes to the Purchasers in the manner contemplated by
the Purchase Agreement or in connection with the Exempt Resales to qualify the
Indenture under the TIA.

          (13) No registration under the Act is required for the sale of the
Series A Notes to the Purchasers as contemplated by the Purchase Agreement or
for the Exempt Resales assuming (i) each Purchaser is a QIB, (ii) the accuracy
of, and compliance with, the Purchasers' representations and agreements
contained in Section 7 of the Purchase Agreement and (iii) the accuracy of the
Issuer's and the Guarantors' representations contained in Sections 5(h), 6(dd),
(ee), (ff), (gg), (hh), (ii) and (jj) of the Purchase Agreement.

          Our opinions set forth in paragraphs (2), (3), (4), (6), (7) and (8)
above are subject to:

          (i)  the effect of any bankruptcy, insolvency, reorganization,
fraudulent conveyance, moratorium or similar laws affecting the enforcement of
creditors' rights generally (including, without limitation, the effect of
statutory or other laws regarding fraudulent transfers or preferential
transfers);

          (ii) rights of acceleration and the availability of equitable
remedies may be limited by equitable principles of general applicability as
applied by the court before which any proceeding therefor may be brought
(regardless of whether such enforcement is considered in a proceeding at law or
in equity);
<PAGE>

Credit Suisse First Boston Corporation
Banc of America Securities LLC
SunTrust Equitable Securities Corporation
Scotia Capital (USA) Inc.
BNY Capital Markets, Inc.
April 11, 2001
Page 7

          (iii)  limitations imposed under applicable law, court decisions or
public policy which limit the enforceability of indemnification or contribution
provisions;

          (iv)   the effect of California Civil Code 1670.5, which provides that
a court may refuse to enforce, or may limit the application of, a contract or
any clause thereof which the court finds as a matter of law to have been
unconscionable at the time it was made;

          (v)    the effect of California statutory provisions and case law that
provide that, in certain circumstances, a surety or guarantor may be exonerated
if the creditor materially alters the original obligation of the principal
without the consent of the guarantor, elects remedies for default which impair
the subrogation rights of the surety or guarantor against the principal or
otherwise takes any action without notifying the guarantor which materially
prejudices the surety or guarantor.  Union Bank v. Gradsky, 265 Cal. App. 2d 40
                                     ---------------------
(1968).  However, there is also authority to the effect that a surety or
guarantor may validly waive such rights if such waivers are expressly set forth
in the guaranty.  Krueger v. Bank of America, 145 Cal. App. 3d 204, 193 Cal.
                  --------------------------
Rptr. 322 (1983) and Section 2856 of the California Civil Code; but see Cathay
                                                                        ------
Bank v. Lee, 14 Cal App. 4th 1533, 18 Cal. Rptr. 2d 420 (1993), in which the
-----------
court held that a waiver of a guarantor's rights must be sufficiently explicit.
Therefore, we express no opinion with respect to the effect of (a)(1) any
modification or amendment of the obligations of the Issuer or any Guarantor
which materially increases such obligations and correspondingly the Guarantors'
obligations in respect thereof; (2) any election of remedies by the Trustee
following the occurrence of an event of default with respect to the obligations
of the Issuer; or (3) any other action by the Trustee which materially
prejudices the Guarantors pursuant to the Operative Documents, if, in any such
instance, such modification, election or action occurs without notice to the
Guarantors and without granting to the Guarantors an opportunity to cure any
default by the Issuer; or (b) any purported waiver by any of the Guarantors that
does not comply with any requirements of explicitness and/or specificity imposed
by any court;

          (vi)   the effect, if any, of limitations arising from certain state
and federal court decisions involving statutes, public policy or principles of
equity and holding that (1) certain covenants and provisions of lending and
security agreements, including those
<PAGE>

Credit Suisse First Boston Corporation
Banc of America Securities LLC
SunTrust Equitable Securities Corporation
Scotia Capital (USA) Inc.
BNY Capital Markets, Inc.
April 11, 2001
Page 8

allowing for acceleration of indebtedness due under debt instruments upon the
occurrence of certain events, impose restrictions or obligations on the borrower
and it cannot be demonstrated that the enforcement of such restrictions or
obligations upon the occurrence of such events is reasonably necessary for the
protection of the lender; and (2) under certain circumstances, purported waivers
of the benefits of statutory provisions or common law rights are unenforceable;

          (vii)   the validity or enforceability of any provision of the
Operative Documents to the extent such provision violates the law of the State
of California that provides that in a contract permitting one party thereto to
recover attorneys' fees, the prevailing party in any action to enforce any
provision of such contract shall be entitled to recover its reasonable
attorneys' fees;

          (viii)  the enforceability under certain circumstances of provisions
waiving unknown future rights and of provisions stating that rights or remedies
are not exclusive, that every right or remedy is cumulative and may be exercised
in addition to or with any other right or remedy, that election of some
particular remedy may be exercised without notice or that failure to exercise or
delay exercising rights or remedies does not operate as a waiver of such right
or remedy;

          (ix)    limitations on the rights or remedies available to any party
insofar as such party may take discretionary action that is arbitrary,
unreasonable or capricious, or is not taken in good faith or in a commercially
reasonable manner, whether or not such action is permitted under the Notes and
the related Subsidiary Guarantees; and

          (x)     the enforceability, under certain circumstances, of provisions
imposing a payment obligation with respect to the Issuer's registration
obligations may be limited by applicable law.

          We further advise you that in our capacity as special counsel for the
Issuer, we have participated in conferences with officers and other
representatives of the Issuer, representatives of the independent public
accountants for the Issuer and representatives of the
<PAGE>

Credit Suisse First Boston Corporation
Banc of America Securities LLC
SunTrust Equitable Securities Corporation
Scotia Capital (USA) Inc.
BNY Capital Markets, Inc.
April 11, 2001
Page 9

Purchasers at which the contents of the Offering Circular and related matters
were discussed. Although we do not pass upon and do not assume any
responsibility for the accuracy, completeness or fairness of the statements
contained in the Offering Circular (except as indicated in paragraph (9) above)
and have made no independent check or verification thereof, on the basis of the
foregoing (relying as to materiality to the extent we deem appropriate upon the
statements of officers and other representatives of the Issuer), no facts have
come to our attention that have caused us to believe that the Offering Circular
(including the documents incorporated by reference, as amended) as of its date
and as of the date hereof contained or contains an untrue statement of a
material fact or omitted or omits to state a material fact necessary in order to
make the statements therein, in light of the circumstances under which they were
made, not misleading (it being understood that we express no opinion on the
financial statements or other financial and statistical (other than industry)
data included or incorporated by reference in the Offering Circular).

         Our opinion in paragraph (10) above as to the lack of required
consents or approvals of governmental authorities is based upon our review of
those statutes, rules and regulations which, in our experience, are normally
applicable to transactions of the type contemplated by the Purchase Agreement
and in addition does not cover federal or state antifraud statutes, rules or
regulations.

          This opinion is rendered to you solely for your benefit in connection
with the Operative Documents and the transactions associated therewith.  This
opinion may not be relied upon by you for any other purpose, or quoted,
circulated, referred or delivered to or relied upon by any other person for any
purpose, without our prior express written consent.

                              Very truly yours,
                               /s/ Riordan & McKinzie

<PAGE>

                                  SCHEDULE A
                                  ----------

                                  Guarantors


Beverly Hills Dialysis Partnership
Carroll County Dialysis Facility, Inc.
Continental Dialysis Center, Inc.
Continental Dialysis Center of Springfield-Fairfax, Inc.
Crescent City Dialysis Partnership
Dialysis Specialists of Dallas, Inc.
East End Dialysis Center, Inc.
Elberton Dialysis Center, Inc.
Flamingo Park Kidney Center, Inc.
Houston Kidney Center/Total Renal Care Integrated Service Network LP
Kenner Regional Dialysis Partnership
Lincoln Park Dialysis Services, Inc.
Mason-Dixon Dialysis Facilities, Inc.
Open Access Sonography, Inc.
Peninsula Dialysis Center, Inc.
Renal Treatment Centers, Inc.
Renal Treatment Centers - California, Inc.
Renal Treatment Centers - Hawaii, Inc.
Renal Treatment Centers - Illinois, Inc.
Renal Treatment Centers - Mid-Atlantic, Inc.
Renal Treatment Centers - Northeast, Inc.
Renal Treatment Centers - Southeast, Inc.
Renal Treatment Centers - West, Inc.
RTC Holdings, Inc.
RTC - Texas Acquisition, Inc.
RTC TN, Inc.
Sunrise Dialysis Partnership
Total Acute Kidney Care, Inc.
Total Renal Care, Inc.
Total Renal Care of Colorado, Inc.
Total Renal Care/Peralta Renal Center Partnership
Total Renal Care/Piedmont Dialysis Center Partnership
Total Renal Care Texas Limited Partnership
Total Renal Care of Utah, L.L.C.
TRC of New York, Inc.
TRC West, Inc.
Total Renal Laboratories, Inc.
Total Renal Research, Inc.
Total Renal Support Services, Inc.
TRC - Indiana LLC
Tri-City Dialysis Center, Inc.
