EXHIBIT 99.1

                CERTIFICATION PURSUANT TO 18 U.S.C. SECTION 1350,

                             AS ADOPTED PURSUANT TO

                  SECTION 906 OF THE SARBANES-OXLEY ACT OF 2002

      In connection with the Quarterly Report on Form 10-Q of The Medicines
Company (the "Company") for the period ended June 30, 2002 as filed with the
Securities and Exchange Commission on the date hereof (the "Report"), I, Clive
A. Meanwell, M.D., Executive Chairman of the Company, hereby certify, pursuant
to 18 U.S.C. Section 1350, that:

            (1)   The Report fully complies with the requirements of Section
                  13(a) or 15(d) of the Securities Exchange Act of 1934; and

            (2)   The information contained in the Report fairly presents, in
                  all material respects, the financial condition and results of
                  operations of the Company.


                                            BY: /S/ CLIVE A. MEANWELL, M.D.
                                            ----------------------------------

Dated: August 14, 2002                      Clive A. Meanwell, M.D.
                                            Executive Chairman