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                                                                     Exhibit 5.1


[LETTERHEAD OF WILMER CUTLER PICKERING HALE AND DORR LLP]


November 18, 2004

Franklin Street Properties Corp.
401 Edgewater Place, Suite 200
Wakefield, MA 01880


      Re:   Registration Statement on Form S-3

Ladies and Gentlemen:

      This opinion is furnished to you in connection with a Registration
Statement on Form S-3 (the "Registration Statement") filed with the Securities
and Exchange Commission (the "Commission") under the Securities Act of 1933, as
amended (the "Securities Act"), for the registration of an aggregate of
25,000,091 shares of Common Stock, $0.0001 par value per share (the "Shares"),
of Franklin Street Properties Corp., a Maryland corporation (the "Company"). All
of the Shares are being registered on behalf of certain stockholders of the
Company (the "Selling Stockholders").

      We are acting as counsel for the Company in connection with the
registration for resale of the Shares. We have examined signed copies of the
Registration Statement as filed with the Commission. We have also examined and
relied upon minutes of meetings of the stockholders and the Board of Directors
of the Company as provided to us by the Company, stock record books of the
Company as provided to us by the Company, the Articles of Organization and
By-Laws of the Company, each as amended to date, and such other documents as we
have deemed necessary for purposes of rendering the opinions hereinafter set
forth.

      In our examination of the foregoing documents, we have assumed the
genuineness of all signatures, the authenticity of all documents submitted to us
as originals, the conformity to original documents of all documents submitted to
us as copies, the authenticity of the originals of such latter documents and the
legal competence of all signatories to such documents.

      Our opinion below, insofar as it relates to the Shares being fully paid,
is based solely on a certificate of the Vice President, Chief Operating Officer,
Secretary and Treasurer of the Company confirming the Company's receipt of the
consideration called for by the applicable resolutions authorizing the issuance
of such Shares.

      We assume that the appropriate action will be taken, prior to the offer
and sale of the Shares, to register and qualify the Shares for sale under all
applicable state securities or "blue sky" laws.

      We express no opinion herein as to the laws of any state or jurisdiction
other than the state laws of the State of Maryland, the General Corporation Law
of the State of Delaware and the federal laws of the United States of America.

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Franklin Street Properties Corp.
November 18, 2004
Page 2


      Based upon and subject to the foregoing, we are of the opinion that the
Shares to be issued by the Company have been duly authorized for issuance and
are validly issued, fully paid and nonassessable.

      It is understood that this opinion is to be used only in connection with
the offer and sale of the Shares while the Registration Statement is in effect.

      Please note that we are opining only as to the matters expressly set forth
herein, and no opinion should be inferred as to any other matters. This opinion
is based upon currently existing statutes, rules, regulations and judicial
decisions, and we disclaim any obligation (after the date of the prospectus
which forms a part of the Registration Statement) to advise you of any change in
any of these sources of law or subsequent legal or factual developments which
might affect any matters or opinions set forth herein.

      We hereby consent to the filing of this opinion with the Commission as an
exhibit to the Registration Statement in accordance with the requirements of
Item 601(b)(5) of Regulation S-K under the Securities Act and to the use of our
name therein and in the related Prospectus under the caption "Legal Matters." In
giving such consent, we do not hereby admit that we are in the category of
persons whose consent is required under Section 7 of the Securities Act or the
rules and regulations of the Commission.



Very truly yours,

WILMER CUTLER PICKERING HALE AND DORR LLP



By:   /s/ Kenneth A. Hoxsie
      ------------------------------
      Kenneth A. Hoxsie, a Partner



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