<PAGE>

                                                                    EXHIBIT 99.1

                                                      NMT MEDICAL, INC.
                                                      27 WORMWOOD STREET
                                                      BOSTON, MA 02210-1625
                                                      (NASDAQ/NMS:NMTI)





AT THE COMPANY                 AT THE FINANCIAL RELATIONS BOARD
-----------------              --------------------------------
Thomas M. Tully                General Info:    Paula Schwartz
President & CEO                Analyst Info:    Brian Gill
(617) 737-0930                 Media Info:      Deanne Eagle
                               (212) 661-8030
FOR IMMEDIATE RELEASE
---------------------
September 13, 1999

            NMT MEDICAL CLOSES $10 MILLION SENIOR DEBT FACILITY WITH
            ---------------------------------------------------------
                          BROWN BROTHERS HARRIMAN & CO.
                          -----------------------------

Boston, MA, September 13, 1999 -- NMT Medical, Inc. (Nasdaq/NMS:NMTI) today
announced the closing of a $10 million senior secured debt facility with Brown
Brothers Harriman & Co.  The facility has a term of three years with interest
payable at Brown Brothers' prime lending rate on U.S. borrowings and an
equivalent market rate on foreign currency borrowings.  Interest will be paid
monthly.  The Company will use $8.0 million of the proceeds and $6.0 million of
its own cash to reduce the principal amount of its current $20 million
subordinated debt obligation with JH Whitney & Company to $6 million.  The
remaining $2.0 million will be available to be drawn down by the Company for
working capital purposes, as needed.  In conjunction with this transaction, the
Company will take an estimated $2.5 million non-cash charge in the third quarter
to accelerate the write-off of the original issue discount paid to JH Whitney &
Company as a part of its subordinated debt agreement of July 1998.

Commenting on the transaction, Thomas M. Tully, President and Chief Executive
Officer of NMT Medical said, "This new financing will be accretive to future
earnings by reducing interest costs, provide increased liquidity, and replace
the monthly convenants required by our current lender with quarterly convenants.
We are very pleased to have Brown Brothers Harriman & Co. as a new business
associate and look forward to a productive relationship."

NMT Medical designs, develops and markets innovative medical devices that
utilize advanced technologies and are delivered by minimally invasive
procedures. The Company's products are designed to offer alternative approaches
to existing complex treatments, thereby reducing patient trauma, shortening
procedure, hospitalization and recovery times, and lowering overall treatment
costs. The Company's medical devices include self-expanding stents, vena cava
filters and septal repair devices. The NMT Neurosciences division serves the
needs of neurosurgeons with a range of products including cerebral spinal fluid
shunts, the Selector(R) Ultrasonic Aspirator, Ruggles(TM)
<PAGE>

Surgical Instruments, the Spetzler(TM) Titanium Aneurysm Clip, and endoscopes
and instrumentation for minimally invasive surgery.





This press release contains "forward-looking statements" within the meaning of
the Private Securities Litigation Reform Act of 1995.  Such forward-looking
statements involve known and unknown risks, uncertainties or other factors which
may cause actual results, performance or achievements of the Company to be
materially different from any future results, performance or achievements
expressed or implied by such forward-looking statements.  Factors that might
cause such a difference include, but are not limited to, those discussed under
the heading "Certain Factors That May Affect Future Results" included in the
Management's Discussion and Analysis of Financial Condition and Results of
Operations in the Company's Annual Report on Form 10-K for the year ended
December 31, 1998.  In addition, statements relating to this financing's impact
on the Company's earnings depends on the actual interest rate payable under the
Brown Brothers' secured debt facility which can fluctuate from time to time and
which could exceed the rate payable under the Company's debt facility with JH
Whitney.

 To receive NMT Medical's latest news release and other corporate documents via
  FAX -- at no cost -- please dial 1-800-PRO-INFO. Enter the Company's symbol
                                     NMTI.

                                     # # #

                                       2