EXHIBIT 5.1

September 21, 2001


Marsh & McLennan Companies, Inc.
1166 Avenue of the Americas
New York, New York 10036-2774


Ladies and Gentlemen:


         I am General Counsel and Senior Vice President of Marsh & McLennan
Companies, Inc., a Delaware corporation (the "Company"). I have acted as counsel
to the Company in connection with the preparation and filing of a Registration
Statement on Form S-8 (the "Registration Statement") under the Securities Act of
1933, as amended, (the "Securities Act") for the registration of 1,000,000
shares of common stock, par value $1.00 per share, including preferred stock
purchase rights (the "Shares"), of the Company relating to the Marsh & McLennan
Companies Special Severance Pay Plan (the "Plan").

         In connection with the foregoing, I or attorneys under my supervision,
have examined the minute books and stock records of the Company, the Certificate
of Incorporation and By-Laws of the Company, the Registration Statement, the
Plan and resolutions duly adopted by the Board of Directors of the Company
relating to the Plan. In addition, I, or attorneys under my supervision, have
reviewed such other documents and instruments and have conferred with various
officers and directors of the Company and have ascertained or verified such
additional facts as deemed necessary or appropriate for the purposes of this
opinion. In this examination I, or attorneys under my supervision, have assumed
the legal capacity of all natural persons, the genuineness of all signatures,
the authenticity of all documents submitted as originals, the conformity to
original documents of all documents submitted as certified, photostatic or
facsimile copies and the authenticity of the originals of such latter documents.

         Based upon the foregoing I am of the opinion that the Shares to be
issued under the Plan have been duly authorized and, when issued and delivered
in accordance with the terms of the Plan, will be legally issued, fully paid and
nonassessable.

         My opinion is limited to matters governed by the laws of the State of
New York and the General Corporation Law of the State of Delaware.

         I hereby consent to the filing of this opinion as an exhibit to the
Registration Statement. In giving this consent, I do not hereby admit that I
come within 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 thereunder.





William L. Rosoff
Page 2 of 2




         This opinion is solely for your benefit in connection with the
Company's offer and sale of the Shares, and is not to be used, circulated,
relied on, quoted or otherwise referred to for any other purpose without my
express written permission.

                                                       Very truly yours,


                                                       /s/William L. Rosoff
                                                       William L. Rosoff
                                                       Senior Vice President and
                                                       General Counsel