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                                                                     EXHIBIT 5.1





                                   August 19, 1996

Transition Systems, Inc.
One Boston Place, 27th Floor
Boston, Massachusetts  02108

Ladies and Gentlemen:

         We are familiar with the Registration Statement on Form S-8 (the "S-8
Registration Statement") filed today by Transition Systems, Inc., a
Massachusetts corporation (the "Company") with the Securities and Exchange
Commission under the Securities Act of 1933, as amended. The S-8 Registration
Statement relates to the proposed offering by the Company of 5,417,925 shares
(the "Shares") of its Common Stock, par value $0.01 per share (the "Common
Stock"), issuable pursuant to the Company's Amended and Restated 1995 Incentive
and Non-Statutory Stock Option Plan (the "Stock Option Plan").

         In arriving at the opinion expressed below, we have examined and relied
upon the following documents:

         (1)  the Amended and Restated Articles of Organization of the Company;

         (2)  the Amended and Restated By-Laws of the Company;

         (3)  the records of meetings and consents of the Board of Directors and
              stockholders of the Company provided to us by the Company; and

         (4)  the Stock Option Plan.

In addition, we have examined and relied upon the originals or copies certified
or otherwise identified to our satisfaction of all such corporate records of the
Company and such other instruments and other certificates of public officials,
officers and representatives of the Company and such other persons, and we have
made such investigations of law, as we have deemed appropriate as a basis for
the opinion expressed below.


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                                                                  EXHIBIT 5.1


Transition Systems, Inc.
August 19, 1996
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Based upon the foregoing, it is our opinion that:

         1.  The Company has corporate power adequate for the issuance of the
Shares in accordance with the S-8 Registration Statement. The Company has taken
all necessary corporate action required to authorize the issuance and sale of 
the Shares. When certificates for the Shares have been duly executed and
counter-signed, and delivered against due receipt of the exercise price for the
Shares as described in the options relating thereto and the Stock Option Plan,
the Shares will be legally issued, fully paid and non-assessable.

         We hereby consent to the filing of this opinion as an exhibit to the
S-8 Registration Statement.


                                             Very truly yours,

                                             FOLEY, HOAG & ELIOT LLP


                                             By: /s/ Robert W. Sweet, Jr.
                                                 -------------------------------
                                                 A Partner