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                                                                     EXHIBIT 5.1

May 20, 2005
                                                             Main (650) 324-7000
                                                              Fax (650) 324-0638

Illumina, Inc.
9885 Towne Centre Drive
San Diego, California 92121

RE:   REGISTRATION STATEMENT ON FORM S-3

Ladies and Gentlemen:

      We have acted as counsel to Illumina, Inc., a Delaware corporation (the
"Company"), in connection with the registration statement on Form S-3 to be
filed by the Company with the Securities and Exchange Commission on or about May
20, 2005 (as may be further amended or supplemented, the "Registration
Statement") for the purpose of registering under the Securities Act of 1933, as
amended (the "Act") an aggregate of 1,579,897 shares of your common stock (the
"Shares") for resale by certain stockholders listed in the Registration
Statement.

      This opinion is being furnished in accordance with the requirements of
Item 16 of Form S-3 and Item 601(b)(5)(i) of Regulation S-K under the Act.

      We have reviewed the Company's organizational documents and the corporate
proceedings taken by the Company in connection with the original issuance of the
Shares. For purposes of rendering this opinion, we have examined the originals
or copies identified to our satisfaction as being true and complete copies of
such corporate records, certificates of officers of the Company and public
officials and such other documents, and have made such other factual and legal
investigations as we have deemed relevant, necessary or appropriate. In such
examination, we have assumed the genuineness of all signatures, the conformity
to original documents of all documents submitted to us as conformed or
photocopies and the authenticity of the originals of such copies. With respect
to agreements and instruments executed by natural persons, we have assumed the
legal competency and authority of such persons. As to facts material to the
opinion expressed herein that were not independently established or verified by
us, we have relied upon oral or written statements and representations of the
Company and others.

      Based on such review, subject to the assumptions stated above and relying
on the statements of fact contained in the documents, instruments, records,
certificates, statements and representations described above, we are of the
opinion that (i) the Shares have been duly authorized and (ii) the Shares are
legally issued, fully paid and non-assessable.

      This opinion is limited to the federal law of the United States of America
and the General Corporation Law of the State of Delaware, and we disclaim any
opinion as to the laws of any other jurisdiction. We further disclaim any
opinion as to any other statute, rule, regulation, ordinance, order or other
promulgation of any other jurisdiction or any regional or local governmental
body or as to any related judicial or administrative opinion.

      We consent to the filing of this opinion as Exhibit 5.1 to the
Registration Statement and to the reference to this firm under the caption
"Legal Matters" in the prospectus that is part of the Registration Statement. In
giving this consent, we do not thereby admit that we are within the category of
persons whose consent is required under Section 7 of the Act, the rules and
regulations of the Securities and Exchange Commission promulgated thereunder, or
Item 509 of Regulation S-K.

      This opinion letter is rendered as of the date first written above and we
disclaim any obligation to advise you of facts, circumstances, events or
developments which hereafter may be brought to our attention and which may
alter, affect or modify the opinion expressed herein. Our opinion is expressly
limited to the matters set forth above and we render no opinion, whether by
implication or otherwise, as to any other matters relating to the Company or the
Shares.

                                                     Very truly yours,

                                                     /s/  Heller Ehrman LLP
