EXHIBIT 5.1

                                December 19, 2003

ARIAD Pharmaceuticals, Inc.
26 Landsdowne Street
Cambridge, MA 02139

Ladies and Gentlemen:

         We have acted as counsel to ARIAD Pharmaceuticals, Inc., a Delaware
corporation (the "Company"), in connection with the preparation and filing with
the Securities and Exchange Commission of a Registration Statement on Form S-3
(the "Registration Statement"), pursuant to which the Company is registering the
issuance under the Securities Act of 1933, as amended, of 7,000,000 shares (the
"Shares") of its common stock, $.001 par value per share (the "Common Stock").
This opinion is being rendered in connection with the filing of the Registration
Statement. All capitalized terms used herein and not otherwise defined shall
have the respective meanings given to them in the Registration Statement.

         In connection with this opinion, we have examined the Company's
certificate of incorporation and restated bylaws, both as amended and currently
in effect; such other records of the corporate proceedings of the Company and
certificates of the Company's officers as we have deemed relevant; and the
Registration Statement and the exhibits thereto.

         In our examination, we have assumed the genuineness of all signatures,
the legal capacity of natural persons, the authenticity of all documents
submitted to us as originals, the conformity to original documents of all
documents submitted to us as certified or photostatic copies and the
authenticity of the originals of such copies.

         Based upon the foregoing, we are of the opinion that the Shares,
provided no stop order shall have been issued by the Commission relating
thereto, when sold pursuant to the Registration Statement (including any
Prospectus Supplement relating thereto), will be validly issued, fully paid and
non-assessable.

         Our opinion is limited to the General Corporation Law of the State of
Delaware (including the applicable provisions of the Delaware Constitution and
the reported judicial decisions interpreting the laws) and the federal laws of
the United States of America, and we express no opinion with respect to the laws
of any other jurisdiction. No opinion is expressed herein with respect to the
qualification of the Shares under the securities or blue sky laws of any state
or any foreign jurisdiction.

         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 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 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 this Firm's name
therein and in the prospectus included therein 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 Securities and Exchange Commission.

                         Very truly yours,

                         /s/ Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C.

                         Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C.

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