<PAGE>
 
                                  EXHIBIT 10


                                   AGREEMENT

     This Agreement is made and entered into as of this 22nd day of April, 1997,
by and between JOHN G. REX-WALLER, an individual residing at 210 Broadway,
Wilmette, Illinois 60091, and NATIONAL SURGERY CENTERS, INC., a Delaware
corporation with its principal office located at 30 South Wacker Drive, Suite
2302, Chicago, Illinois 60606.

                                    RECITALS
                                    --------

     WHEREAS, John G. Rex-Waller (hereinafter "Rex-Waller") was hired by
National Surgery Centers, Inc. (hereinafter "NSC") on or about July 1991; served
as NSC's Chief Financial Officer from July 1991 until January 1996; has served
as NSC's Treasurer and Secretary since July 1991; and has also served as NSC's
Executive Vice President since January 1996; and

     WHEREAS, NSC and Rex-Waller have agreed that Rex-Waller's employment with
NSC shall terminate as of April 22, 1997 (the "Termination Date").

     NOW, THEREFORE, Rex-Waller and NSC do hereby voluntarily enter into this
Agreement (hereinafter "Agreement") in compromise and settlement of all claims,
actions and disputes between the parties.

                              TERMS OF AGREEMENT
                              ------------------

     In consideration of the premises, the recitals of which are incorporated in
and by this reference made a part of this Agreement, the following mutual
covenants, and other good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, Rex-Waller and NSC hereby agree as
follows:

     1.   It is understood and agreed that nothing contained in this Agreement
shall be construed as an admission of liability or violation of any law, public
policy or contract on the part of either party, all such liability being
specifically denied. Rex-Waller represents and warrants that he has filed no
complaints or charges with any court or administrative agencies against NSC.

     2.   Rex-Waller promises and agrees as follows:

          a.   he will resign as (i) an employee of NSC and (ii) a member of the
Board of Directors of NSC, both effective as of the Termination Date, and shall
execute and return, on the Termination Date, a resignation in the form attached
hereto and made a part hereof as Exhibit 1.

          b.   he will execute and return, on the Termination Date, the General
Release And Covenant Not To Sue in the form attached hereto and made a part
hereof as Exhibit 2.

          c.   he shall not, either directly or indirectly, for a period of two
years following the Termination Date engage, anywhere in the United States of
America, in the development, management, or direct or indirect ownership of
other ambulatory surgery centers or in any other business which would be
competitive with the business being conducted by NSC on such Termination Date
(the "Restricted Business"), or acquire or retain any direct or indirect
financial interest in any business which is so engaged; provided, however, that
Rex-Waller may invest in, or be an employee, director, partner or shareholder
of, or a consultant to (i) any company (including, without limitation, a

                                    Page 1
<PAGE>
 
company whose business is physician practice management) which incidentally
includes the Restricted Business, provided that Rex-Waller does not participate
in any of such company's activities that involve the Restricted Business which
take place within a twenty-five mile radius of any facility operated, managed,
or being developed by NSC on such Termination Date or (ii) any firm or entity
which invests in companies ("Portfolio Companies") which engage in the
Restricted Business; provided, however, that Rex-Waller may not participate in
any of such Portfolio Companies' activities that involve the Restricted Business
unless (i) the Portfolio Company only incidentally includes the Restricted
Business and (ii) such activities do not take place within a twenty-five mile
radius of any facility operated, managed, or being developed by NSC on such
Termination Date . Notwithstanding the foregoing, nothing contained in this
paragraph 2c shall prohibit Rex-Waller from being (A) a shareholder in a mutual
fund or a diversified investment company or (B) a passive owner of not more than
two percent of the outstanding stock of any class of a company so long as Rex-
Waller has no active participation in the business of such company.

          d.   for a period of two years from the Termination Date, he shall
not, nor shall he cause any person to, solicit to employ any of the current
officers or employees of NSC, without obtaining the prior written consent of
NSC.

     3.   NSC promises and agrees as follows:

          a.   to provide Rex-Waller with a payment in the amount of $50,000, on
January 2, 1998, for his consulting services in connection with the possible
acquisition by NSC of two additional ambulatory surgery centers.

          b.   to make available to Rex-Waller continued COBRA health insurance
coverage, ending when Rex-Waller is no longer eligible for continued coverage,
as defined under COBRA.

          c.   to execute and return, on the Termination Date, the General
Release And Covenant Not To Sue in the form attached hereto and made a part
hereof as Exhibit 3.

          d.   in the event a Change in Control of NSC, as defined herein,
occurs prior to March 31, 1998, NSC shall pay to Rex-Waller within 30 days of
such Change of Control, in a lump sum an amount equal to the sum of $284,000,
representing (i) Rex-Waller's annual base salary for 1996 and (ii) the annual
bonus paid to Rex-Waller for 1996 by NSC. For purposes of this paragraph 3d, a
"Change in Control" shall mean a change in control of a nature that would be
required to be reported in response to Item 6(e) of Schedule 14A of Regulation
14A promulgated under the Securities Exchange Act of 1934; provided that,
without limitation, such a Change in Control shall be deemed to have occurred if
during any period of two consecutive years subsequent to November 9, 1995,
individuals who at the beginning of such period constitute members of the board
of directors cease for any reason to constitute at least a majority thereof,
unless the nomination or election of each director who was not a director at the
beginning of the period was approved by a vote of a majority of the directors
still in office at the time of such nomination or election who were directors at
the beginning of the period. If a tax is imposed pursuant to Section 4999 of the
Internal Revenue Code, or successor provision of like import, upon payments due
under this paragraph 3d, Rex-Waller shall be paid an additional amount
calculated so as to provide Rex-Waller, after he has paid the tax imposed by
Section 4999 of the Code, with the same compensation he would have received had
no tax been imposed by Section 4999.

     4.   NSC represents and warrants that (a) the Compensation Committee of
NSC's Board of Directors (the "Committee") (i) accelerated the vesting of all of
Rex-Waller's options under the Stock Option Plan to the Termination Date, (ii)
pursuant to clause (iii) of Section 6 of the Stock Option Plan, extended to
December 31, 1997 the period during which Rex-Waller may exercise options to
purchase 90,000 shares of NSC's common stock ("Common Stock") at an exercise
price of $27.50 per share, which options were granted to Rex-Waller on June 21,
1996, (iii) approved Rex-Waller's exercise within three months following the
Termination Date of options to purchase up to 182,500 shares of Common Stock by
his tendering to NSC shares of Common Stock owned by him in full payment of the
exercise price of such shares in accordance with clause (i) of Section 8 of the
Stock Option Plan and (iv) approved Rex-Waller's

                                    Page 2
<PAGE>
 
exercise within three months following the Termination Date of options to
purchase up to 182,500 shares of Common Stock by his delivery to NSC in
accordance with clause (ii) of Section 8 of the Stock Option Plan of an
irrevocable written notice instructing NSC to deliver the shares of Common Stock
being purchased to a broker selected by NSC, subject to such broker's written
guarantee to deliver cash to NSC equal to the aggregate exercise price for the
shares being purchased, (b) at the date hereof NSC has, and at the time of the
exercise of such options and the sale of the shares of Common Stock underlying
such options NSC will continue to have, an effective registration statement on
Form S-8 registering the sale of such shares under the Securities Act of 1933,
as amended, and (c) NSC has not and will not require Rex-Waller to deliver an
investment representation with respect to the shares of Common Stock to be
acquired upon his exercise of such options and has not and will not impose any
restrictions upon Rex-Waller's exercise of such options or his sale of such
shares.

     5.   NSC agrees not to criticize or disparage Rex-Waller, either within or
without NSC, either orally or in writing, and not to authorize or condone any
such criticism or disparagement; provided, that nothing contained in this
paragraph 5 shall preclude NSC from making any statement to its attorneys on a
confidential basis or from making any statement in good faith which is required
by law, regulation or order of any court or regulatory commission, department or
agency. Likewise, Rex-Waller agrees not to criticize or disparage NSC, NSC's
officers or agents, or NSC's business or employment practices, either orally or
in writing; provided, that nothing contained in this paragraph 5 shall preclude
Rex-Waller from making any statement to his attorneys on a confidential basis or
from making any statement in good faith which is required by law, regulation or
order of any court or regulatory commission, department or agency. Rex-Waller
further agrees that he will not intentionally or knowingly instigate, cause,
advise or encourage any person, group of persons or entity to file suit against
NSC; and that he will not join in, or if named will opt out of, any suit that
may be filed against NSC based on any facts and events occurring up to and
including the date of this Agreement.

     6.   Rex-Waller promises and agrees that he will continue to hold in
strictest confidence and will not use or disclose to any person, firm or
corporation, without the prior, written authorization of an officer of NSC, any
of NSC's trade secrets and other proprietary and confidential business
information, or trade secrets and other proprietary and confidential business
information of a third party provided to NSC. Rex-Waller understands that this
agreement applies to computerized as well as written information. It is
expressly understood, however, that the obligations of this paragraph 6 shall
(a) only apply for as long as and to the extent that said trade secrets and
other proprietary and confidential business information have not become
generally known to or available for use by the public other than by an act or
omission of Rex-Waller, and (b) not apply if such disclosure is required by any
law, regulation or order of any court or regulatory commission, department or
agency.. The term "trade secrets and other proprietary and confidential business
information" means business and technical information treated as confidential by
NSC, including, without limitation, information relating to sales, customers and
customer lists, financial and marketing data, business plans, and personnel and
personnel related practices, policies and procedures, all of which Rex-Waller
became aware of as a result of his employment with NSC.

     7.   Rex-Waller acknowledges NSC's title to, and exclusive right to
possession and use of, all reports, papers or documents of NSC, or any copies,
abstracts or summaries thereof, including drawings, which came into his
possession during his employment with NSC. Rex-Waller also acknowledges that he
has returned to NSC all such reports, papers or documents, except for those
reports, papers or documents which NSC has advised Rex-Waller, in writing, he
may keep. If NSC later believes that Rex-Waller has not complied with the
requirements of this paragraph 7 in any respect, it shall request in writing and
with specificity the report, paper or document to be returned by Rex-Waller.
Rex-Waller shall have a reasonable period of time following such written request
to return the report, paper or document so requested, but if Rex-Waller does not
have control of, or access to, such report, paper or document, he shall notify
NSC of such fact in writing and, upon such notification, he shall have complied
fully with the requirements of this paragraph 7. This provision is not intended
to require Rex-Waller to turn over to NSC information acquired by Rex-Waller
from sources not connected or related to NSC or created by Rex-Waller for his
own use or for use in his profession and which is not a trade secret of NSC or
proprietary to NSC.

                                    Page 3
<PAGE>
 
     8.   a.   NSC agrees to indemnify, defend and hold Rex-Waller harmless from
any liability arising out of actions taken by Rex-Waller as an employee of NSC,
including without limitation liability brought about or con tributed to by
actions covered by NSC's professional liability policy, and reimburse Rex-
Waller, within 15 days following his request therefor, for his reasonable
expenses incurred in the defense thereof, such as transportation, meals, lodging
and lost wages. Rex-Waller shall advise NSC promptly of any proceeding in which
he is named a party defendant, reasonably cooperate with NSC in connection
therewith, and not settle any proceeding without the prior, written consent of
NSC. Rex-Waller also agrees to reasonably cooperate with NSC in any proceeding
against NSC relating to or concerning matters or issues involving his employment
at NSC, even if he is not named a party defendant in such proceeding, and,
provided that Rex-Waller and NSC are not adverse parties in such proceeding, NSC
will provide legal representation to Rex-Waller at NSC's expense and selection
and reimburse Rex-Waller, within 15 days following his request therefor, for his
reasonable expenses incurred in connection therewith, such as transportation,
meals, lodging and lost wages.

          b.   Notwithstanding anything to the contrary contained in paragraph
8a hereof, NSC agrees that the limitation of liability now existing in favor of
Rex-Waller contained in Article Tenth of NSC's Certificate of Incorporation and
all rights to indemnification now existing in favor of Rex-Waller contained in
Article Tenth of NSC's Certificate of Incorporation and Article VII Section 5 of
NSC's By-laws, in each case as in effect on the date hereof, shall not be
amended in any manner that would adversely affect the rights of Rex-Waller,
unless such amendment is required by law. To the extent permitted by the laws of
the State of Delaware, such indemnification shall be mandatory and not
permissive and NSC shall advance all fees and expenses (including attorneys' and
paralegals' fees for legal counsel retained by NSC on Rex-Waller's behalf and
other costs and expenses) as incurred in connection with such indemnification.

          c.   Notwithstanding anything to the contrary contained in paragraph
8a hereof, pursuant to the rights to indemnification referred to in paragraph 8b
hereof, NSC agrees to indemnify and hold harmless Rex-Waller and his successors
to the fullest extent permitted by the laws of the State of Delaware with
respect to any claim arising at any time out of any event, action or omission
related to or in connection with Rex-Waller having been (i) a director, officer
or employee of NSC or having served as a director of Endoscopy Center
Affiliates, Inc., a Delaware corporation and a wholly-owned subsidiary of NSC,
or as a director or officer of another corporation or other organization at the
request of NSC, and (ii) the trustee and a fiduciary of the National Surgery
Centers, Inc. 401(k) Plan to the fullest extent permitted by the Employee
Retirement Income Security Act of 1974, as amended. This indemnification shall
continue in full force and effect for a period of not less than the duration of
all statutes of limitations applicable to such matters (or in the case of
events, actions or omissions giving rise to matters which have not been resolved
prior to the expiration of such period, until such matters are finally
resolved). Without limiting the foregoing, NSC shall periodically advance all
fees and expenses (including the fees of attorneys' and paralegals retained by
NSC on Rex-Waller's behalf and other costs and expenses) as incurred with
respect to the foregoing to the fullest extent permitted by the laws of the
State of Delaware, and Rex-Waller shall be defended by counsel of NSC's choice.

     9.   References and inquiries concerning Rex-Waller's employment at NSC
shall be directed only to E. Timothy Geary, NSC's President, or his successor.

     10.  Rex-Waller represents and warrants that, prior to and including the
date of this Agreement, no claim, demand or cause of action which is the subject
of this Agreement has been assigned or transferred to any other person or
entity; that no other person or entity has or has had any interest in any claim,
demand or cause of action which is the subject of this Agreement; that no person
or entity has or has had any subrogated interest in any claim, demand, cause of
action or payment of settlement funds which is the subject of this Agreement;
and that he has the sole right and authority to execute this Agreement.

     11.  This Agreement shall be binding in all respects upon, and inure to the
benefit of, any and all agents,

                                    Page 4
<PAGE>
 
administrators, executors, heirs, successors, assigns, transferees and parties
claiming under and through Rex-Waller, and any of them. This Agreement shall
also be binding in all respects upon, and inure to the benefit of, NSC's
successors and assigns and all parties claiming under and through NSC.

     12.  This Agreement may be pleaded as a full and complete defense to, and
may be used as a basis for an injunction against, any action at law, proceeding
in equity or any other judicial or non-judicial proceeding that either party may
institute, prosecute or maintain, or continue to prosecute or maintain, in
breach thereof.

     13.  This Agreement contains the entire agreement between Rex-Waller on the
one hand and NSC on the other. The parties acknowledge that they have not relied
upon any promises made by a representative of the other, other than those
specifically contained herein, as an inducement to sign this Agreement or the
Exhibits to this Agreement. No change, modification or waiver of any provision
of this Agreement shall be valid or binding unless it is in writing and signed
by the party to be bound. This Agreement supersedes the Employment Agreement
dated as of November 9, 1995, between NSC and Rex-Waller and such agreement
shall be of no further force or effect.

     14.  If any provision of paragraph 2c of this Agreement, as applied to any
party or to any circumstances, is adjudged by a court to be invalid or
unenforceable, the same will in no way affect any other provision of the said
paragraph 2c or any other part of this Agreement, the application of such
provision in any other circumstances or the validity or enforceability of this
Agreement. If any such provision, or any part thereof, is held to be
unenforceable because of the duration of such provision or the area covered
thereby, the parties agree that the court making such determination will have
the power to reduce the duration and/or area of such provision, and/or to delete
specific words or phrases, and in its reduced form such provision will then be
enforceable and will be enforced.

     15.  Rex-Waller represents and warrants that he was given a reasonable
amount of time (14 days) to consider the terms of this Agreement and has had an
opportunity to inquire into all material facts pertinent to this Agreement; and
that he is represented by counsel (Sidley & Austin) and has consulted with
counsel prior to signing this Agreement.

     16.  The undersigned represent and warrant that they are fully authorized
and competent to enter into the terms of and execute this Agreement, and that
they do so knowingly and voluntarily.

                                    Page 5
<PAGE>
 
IT IS SO AGREED:



-----------------------------------------------     Date:
John G. Rex-Waller                                       -----------------------

NATIONAL SURGERY CENTERS, INC.



By:  
       ----------------------------------------     Date:
                                                         -----------------------
Title: 
       ----------------------------------------

                                    Page 6
<PAGE>
 
                                   EXHIBIT 1


                                  RESIGNATION
                                  -----------

 
     I, John G. Rex-Waller, do hereby resign as (i) an employee (including my
positions as Executive Vice President, Secretary, and Treasurer) and a member of
the Board of Directors of National Surgery Centers, Inc., a Delaware corporation
("NSC"), (ii) an officer and a member of the Board of Directors of each of the
subsidiaries of NSC in which I am currently serving in either or both of these
capacities (including, without limitation, as an officer and a member of the
Board of Directors of Endoscopy Center Affiliates, Inc., a Delaware corporation
and wholly-owned subsidiary of NSC), and (iii) the trustee and a fiduciary of
the National Surgery Centers, Inc. 401(k) Plan, effective on the date hereof.



Dated this 22nd day of April, 1997

                                          --------------------------------------
                                                    John G. Rex-Waller
<PAGE>
 
                                   EXHIBIT 2
                                   ---------

                    GENERAL RELEASE AND COVENANT NOT TO SUE
                    ---------------------------------------

     In consideration of the good and valuable consideration set forth in the
Agreement, dated April 22, 1997, between John G. Rex-Waller ("Rex-Waller") and
National Surgery Centers, Inc., the receipt and sufficiency of which are hereby
acknowledged, Rex-Waller agrees as follows:

     A.   Rex-Waller agrees to and does hereby release, acquit and forever
          discharge National Surgery Centers, Inc.; its successors and assigns;
          and its present and former agents, directors, employees, officers and
          representatives (hereinafter collectively referred to as the "Released
          Parties"), of and from any and all claims, demands and causes of
          action of every kind or character, in every proceeding whatsoever,
          known or unknown, now existing or which may hereafter arise, by reason
          of any act or omission on the part of the Released Parties occurring
          at any time up to and including the date of execution of this General
          Release and Covenant Not to Sue. Without restricting the generality of
          the foregoing, Rex-Waller agrees to and does hereby release, acquit
          and forever discharge the Released Parties of and from any and all
          claims, demands and causes of action, known or unknown, arising out of
          or in any way connected with or relating to his employment and
          termination of employment with National Surgery Centers, Inc., from
          the beginning of said employment up to and including the date of
          execution of this General Release and Covenant Not to Sue, including
          but not limited to claims, demands and causes of action based in whole
          or in part on (i) breach of contract, including breach of the
          Employment Agreement, dated November 9, 1995, by and between Rex-
          Waller and National Surgery Centers, Inc.; (ii) impairment of economic
          opportunity, intentional infliction of emotional harm, negligent
          infliction of emotional distress, defamation, interference with
          contractual relations, interference with prospective economic
          advantage, or other tort; (iii) the Age Discrimination in Employment
          Act, the Americans With Disabilities Act, the Equal Pay Act, the
          Employee Retirement Income Security Act, Title VII of the Civil Rights
          Act, the Civil Rights Act of 1991, the State of Illinois Human Rights
          Act, the Cook County Human Rights Ordinance, and any other federal,
          state or municipal statute or ordinance relating to discrimination in
          employment, and (iv) any other legal restrictions prohibiting, in
          whole or in part, the actual or constructive discharge of employees.
          Notwithstanding anything to the contrary set forth in this General
          Release and Covenant Not to Sue, this release shall not apply to, or
          release the Released Parties from, any obligation contained in the
          Agreement dated April 22, 1997, between Rex-Waller and National
          Surgery Centers, Inc. or any obligation under any benefit plan or
          arrangement of National Surgery Centers, Inc., including National
          Surgery Centers, Inc.'s Amended and Restated 1992 Stock Option Plan,
          which entitles Rex-Waller to the receipt of any benefit, or any claim
          for benefits that Rex-Waller may have under any ERISA covered employee
          benefit plan in which he was a participant.
<PAGE>
 
     B.   Rex-Waller agrees that he will not commence or bring any action at
          law, proceeding in equity, or any other judicial or non-judicial
          proceeding against the Released Parties relating, in whole or in part,
          to his employment and termination of employment with National Surgery
          Centers, Inc.. Notwithstanding anything to the contrary set forth in
          this General Release and Covenant Not to Sue, this covenant not to sue
          shall not apply to, or release the Released Parties from, any
          obligation contained in the Agreement dated April 22, 1997, between
          Rex-Waller and National Surgery Centers, Inc. or any obligation under
          any benefit plan or arrangement of National Surgery Centers, Inc.,
          including National Surgery Centers, Inc.'s Amended and Restated 1992
          Stock Option Plan, which entitles Rex-Waller to the receipt of any
          benefit, or any claim for benefits that Rex-Waller may have under any
          ERISA covered employee benefit plan in which he was a participant.

     Rex-Waller acknowledges that he was advised in writing to consult with an
attorney prior to executing this General Release And Covenant Not To Sue, and
that he was given a reasonable amount of time to consider the terms of this
General Release And Covenant Not To Sue.


-----------------------------------        Date: 
John G. Rex-Waller                              ------------------


Subscribed and sworn to
before me this __ day of
April, 1997.

------------------------
      Notary Public

                                    Page 2
<PAGE>
 
                                   EXHIBIT 3

                    GENERAL RELEASE AND COVENANT NOT TO SUE
                    ---------------------------------------

     In consideration of the good and valuable consideration set forth in the
Agreement, dated April 22, 1997, between John G. Rex-Waller ("Rex-Waller") and
National Surgery Centers, Inc., a Delaware corporation ("NSC"), the receipt and
sufficiency of which are hereby acknowledged, NSC agrees as follows:

     A.   NSC, on behalf of itself and each of its divisions, subsidiaries,
          affiliates and other related entities (whether or not such entities
          are wholly owned) and its past, present and future directors,
          officers, employees and agents, and the predecessors, successors and
          assigns of each of them, agrees to and does hereby release, acquit and
          forever discharge Rex-Waller and anyone claiming through him
          including, but not limited to, his past, present and future agents,
          family members, representatives, heirs, executors and administrators,
          or the predecessors, successors and assigns of each of them
          (hereinafter collectively referred to as the "Released Parties"), of
          and from any and all claims, demands and causes of action of every
          kind or character, in every proceeding whatsoever, known or unknown,
          now existing or which may hereafter arise, by reason of any act or
          omission on the part of the Released Parties occurring at any time up
          to and including the date of execution of this General Release and
          Covenant Not to Sue. Without restricting the generality of the
          foregoing, NSC agrees to and does hereby release, acquit and forever
          discharge the Released Parties of and from any and all claims, demands
          and causes of action, known or unknown, arising out of or in any way
          connected with or relating to Rex-Waller's employment and termination
          of employment with NSC from the beginning of said employment up to and
          including the date of execution of this General Release and Covenant
          Not to Sue, including but not limited to claims, demands and causes of
          action based in whole or in part on breach of contract, including
          breach of the Employment Agreement, dated November 9, 1995, by and
          between Rex-Waller and NSC. Notwithstanding anything to the contrary
          set forth in this General Release and Covenant Not to Sue, this
          release shall not apply to, or release the Released Parties from, any
          obligation contained in the Agreement dated April 22, 1997, between
          Rex-Waller and NSC.

     B.   NSC agrees that it will not commence or bring any action at law,
          proceeding in equity, or any other judicial or non-judicial proceeding
          against the Released Parties relating, in whole or in part, to Rex-
          Waller's employment and termination of employment with NSC.
          Notwithstanding anything to the contrary set forth in this General
          Release and Covenant Not to Sue, this release shall not apply to, or
          release the Released Parties from, any obligation contained in the
          Agreement dated April 22, 1997, between Rex-Waller and NSC.


NATIONAL SURGERY CENTERS, INC.


By:                                             Date: 
       ----------------------------------            ------------------
Title:  
       ----------------------------------

SUBSCRIBED and sworn to
before me this ___ day of
April, 1997.

-------------------------
      Notary Public