Exhibit 5

75 State Street                WARNER & STACKPOLE LLP  Telephone: (617) 951-9000
Boston, Massachusetts 02109    COUNSELLORS AT LAW          Fax: (617) 951-9151

                                  June 5, 1998

Vertex Pharmaceuticals Incorporated
130 Waverly Street
Cambridge, MA  02139-4211

Ladies and Gentlemen:

         We have acted as counsel to Vertex Pharmaceuticals Incorporated, a
Massachusetts corporation (the "Company"), in connection with the preparation
and filing with the Securities and Exchange Commission (the "Commission") of a
Registration Statement on Form S-8 (the "Registration Statement") registering
for issuance by the Company, upon exercise of options awarded and to be awarded
under the Company's 1996 Stock and Option Plan (the "Plan"), an additional
1,250,000 shares (the "Shares") of the Common Stock, $.01 par value per share
("Common Stock"), of the Company. These shares are in addition to 2,000,000
shares of Common Stock issuable under the Plan registered on Form S-8,
Registration No. 333-27011 filed with the Commission on May 13, 1997.

         We have examined the Registration Statement, the Restated Articles of
Organization of the Company and such other documents and records of the Company
as we have deemed necessary for the purpose of this opinion.

         In our examination of the foregoing documents, we have assumed the
genuineness of all signatures and 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 latter documents.

         We are members of the bar of the Commonwealth of Massachusetts, and we
express no opinion as to any matters insofar as any laws other than Federal laws
and the laws of the Commonwealth of Massachusetts may be applicable.

         We assume for purposes of this opinion that the grants of options under
the Plan have been or will be made in accordance with the terms and conditions
of the Plan.


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                             WARNER & STACKPOLE LLP

Vertex Pharmaceuticals Incorporated
June 5, 1998
Page 2

         Based upon the foregoing, we are of the opinion that the Shares are
duly authorized and reserved for issuance pursuant to the terms of the Plan, and
upon (i) the effectiveness of the Registration Statement, (ii) payment for the
Shares in accordance with the terms of the Plan and (iii) the issuance of
certificates therefor by the Company, the Shares will be validly issued, fully
paid and non-assessable.

         We hereby consent to the filing of this opinion as an exhibit to the
Registration Statement.

                                                     Very truly yours,

                                                     /s/ Warner & Stackpole LLP



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