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                                                                   EXHIBIT 10.12

                                 WAIVER NO. 2
                                 ------------

     This Waiver No. 2, made as of November 12, 1999 (this "Waiver"), is by and
between NMT Medical, Inc. (the "Company"), on the one hand, and Whitney
Subordinated Debt Fund, L.P., on the other hand.  Capitalized terms used herein
and not otherwise defined have the meanings assigned to such terms in the
Purchase Agreement (as defined below).

                              W I T N E S S E T H
                              -------------------

     WHEREAS, the Company and the Purchaser are parties to the Subordinated Note
and Common Stock Purchase Agreement, dated as of July 8, 1998, as amended by
Amendment No. 1 dated as of April 14, 1999 and Amendment No. 2 dated as of
September 13, 1999 (as so amended, the "Purchase Agreement"), regarding the
Company's subordinated notes due September 30, 2003;

     WHEREAS, the Company has requested the Purchaser to waive compliance with
certain covenants contained in the Purchase Agreement for the fiscal quarter
ended September 30, 1999.

     NOW, THEREFORE, in consideration of the premises and mutual agreements
contained herein, and other good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, the parties hereto agree as
follows:

     1.  The Purchaser hereby waives compliance by the Company with the
provisions of Section 9.8(a), 9.8(b) and 9.8(d) of the Purchase Agreement solely
with respect to the fiscal quarter ended September 30, 1999.  Such waiver shall
not apply to any other covenants or to any period other than the fiscal quarter
ended September 30, 1999.

     2.  This Waiver may be signed in counterparts, and by the various parties
on separate counterparts.  Each set of counterparts which contains the signature
of each of the parties shall constitute a single instrument with the same effect
as if the signature thereto were upon the same instrument.  The parties hereto
agree that each party shall accept facsimile signatures as legally sufficient,
binding and admissible evidence of the execution of this Waiver.

     3.  Except as expressly modified by this Waiver, all of the terms and
provisions of the Purchase Agreement and the Notes shall continue in full force
and effect and all parties hereto shall be entitled to the benefits thereof.

     4.  This Waiver shall be governed by and construed in accordance with the
laws of the State of New York, without regard to the principals of conflict of
laws of such state.
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     IN WITNESS WHEREOF, the parties hereto have caused this Waiver to be signed
by their respective duly authorized officers as of the date first written above.

                         NMT MEDICAL, INC.


                         By: /s/ William J. Knight
                             -----------------------------------------
                             Name:  William J. Knight
                             Title: Vice President of Finance and
                                    Administration and Chief Financial
                                    Officer

                         WHITNEY SUBORDINATED DEBT FUND, L.P.


                         By: /s/ Jeffrey R. Jay
                             -----------------------------------------
                             Name:  Jeffrey R. Jay
                             Title: A General Partner