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                                                                    EXHIBIT 4.12
 
                               AMENDMENT NO. 1 TO
                         EXECUTIVE STOCK AGREEMENT AND
                        EMPLOYMENT AGREEMENT AND CONSENT
                        --------------------------------


     This Amendment No. 1 to Executive Stock Agreement and Employment Agreement
and Consent (this "Amendment") is entered into as of this 21st day of August,
1998, between Focal Communications Corporation, a Delaware corporation (the
"Company"), John R.Barnicle ("Executive"), Brian F. Addy, Joseph Beatty, Robert
C. Taylor, Jr., Madison Dearborn Capital Partners, L.P., Frontenac VI, L.P. and
Battery Ventures III, L.P. (collectively with Executive, the "Stockholders").
Capitalized terms not otherwise defined in this Agreement are used herein with
the meanings assigned to such terms in the Stock Purchase Agreement, dated
November 27, 1996, by and among the Company and the other parties thereto (as
amended, the "Stock Purchase Agreement") or the Executive Stock Agreement (as
hereinafter defined), as the case may be.

     WHEREAS, the Company and the Executive wish to amend the provisions of the
Executive Stock Agreement and Employment Agreement, dated November 27, 1996, by
and between the Company and the Executive (the "Executive Stock Agreement") as
provided in paragraph 1 of this Amendment;

     WHEREAS, the Stockholders collectively own all of the Institutional
Investor Stock and at least a majority of the Executive Stock; and

     WHEREAS, the Stockholders wish to consent to the amendment (as provided in
paragraph 1 of this Amendment) for purposes of the Stock Purchase Agreement;

     NOW, THEREFORE, for good and valuable consideration, the receipt and
sufficiency of which is hereby acknowledged, the parties to this Amendment
hereby agree as follows:

     1.  Amendments to Executive Stock Agreement.  Pursuant to Section 9(h) of
         ---------------------------------------                              
the Executive Stock Agreement:

         (a) Paragraph 2(d) of the Executive Stock Agreement is hereby amended
     and restated to read in its entirety as follows:

         "(d) Acceleration upon a Public Offering. At the closing 
              -----------------------------------      
         of a Public Offering, if the Executive is then employed by
         the Company, there will vest the number of Unvested Shares
         which were scheduled to vest within 12 months following such
         closing, and on each anniversary of the Closing hereunder
         following the closing of a Public Offering, if the Executive
         is then employed by the Company, there will vest 20% of the
         original Unvested Shares (so that the
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         vesting schedule set forth in paragraph 2(a) above shall have
         been effectively accelerated by one year)."

         (b)   The first two sentences of paragraph 2(e) of the Executive Stock
     Agreement are hereby deleted and replaced with the following:

         "(e)  Acceleration upon Death or Disability. If Executive's
               -------------------------------------  
         employment with the Company or any of its Subsidiaries
         terminates by reason of Executive's death or Disability, all
         Unvested Shares shall become Vested Shares."

         (c)   Paragraph 2(f) of the Executive Stock Agreement is hereby amended
     by adding the following sentence at the end of such section:

         "Any Unvested Shares which the Company (or its assignees
         pursuant to paragraph 3(e) hereof) has elected to repurchase
         in the Repurchase Notice provided for in paragraph 3(b) below
         but for which a closing in accordance with paragraph 3(g)
         below has not occurred within the time frame set forth in
         paragraph 3(g) (as modified by paragraph 3(h), if applicable
         or extended by request of or agreement by the Executive) for
         a reason other than Executive's failure to perform his
         obligations under this paragraph 3 shall thereafter be deemed
         Vested Shares for all purposes of this Agreement and the
         Stockholders Agreement."

         (d)   Paragraph 3(c)(ii) of the Executive Stock Agreement is hereby
     amended and restated in its entirety to read as follows:

         "(ii) The Repurchase Price for shares of Vested Executive
         Stock repurchased hereunder shall be the fair market value of
         such shares on the date of the Repurchase Notice (determined
         according to the method set forth in paragraph 3(d) below)."

         (e)   Paragraph 3(h) of the Executive Stock Agreement is hereby amended
     and restated in its entirety as follows:

         "(h)  Restrictions. All repurchases of Executive Stock by the
               ------------
         Company shall be subject to applicable restrictions contained
         in the Delaware General Corporation Law and in the Company's
         and its Subsidiaries debt and equity financing agreements. If
         any such restrictions prohibit the repurchase of Executive
         Stock hereunder which the Company is otherwise entitled or
         required to make, the time periods provided in this paragraph
         3 shall be suspended for a period of time not exceeding six
         months in the aggregate, and the Company may make such
         repurchases (subject to compliance with

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          paragraph 3(b) and the other sections of this paragraph 3,
          as modified by this paragraph 3(h), if applicable) as soon
          as it is permitted to do so under such restrictions, unless
          by such time such Repurchase Option has terminated pursuant
          to paragraph 3(i) or has been deemed forfeited pursuant to
          this paragraph 3(h). Notwithstanding anything in this
          paragraph 3 to the contrary, any repurchase of Executive
          Stock by the Company and/or its assignees hereunder for
          which a closing in accordance with paragraph 3(g) below has
          not occurred within the time frame set forth in paragraph
          3(g) (as modified by the paragraph 3(h), if applicable, or
          extended by request of or agreement by the Executive) for a
          reason other than Executive's failure to perform his
          obligations under this paragraph 3 shall be deemed forfeited
          and neither the Company nor its assignees shall have any
          right to repurchase such shares hereunder."

          (f)  Paragraph 3(i) of the Executive Stock Agreement is hereby amended
     and restated in its entirety to read as follows:

          "(i) Termination of Repurchase Option. Unless terminated
               --------------------------------
          sooner pursuant to the terms of the following sentence, all
          rights under this paragraph 3 of the Company and/or its
          assignees to repurchase Vested Executive Stock (but not
          Unvested Shares), whether or not exercised, shall terminate
          at the closing of a Public Offering. To the extent not
          terminated sooner pursuant to the terms of the immediately
          preceding sentence, all rights under this paragraph 3 of the
          Company and/or its assignees to repurchase Executive Stock
          (whether Vested Shares or Unvested Shares), whether or not
          exercised, shall terminate upon a Qualified Sale of the
          Company."

          (h)  The definition of "Noncompete Compensation" set forth in the
     fourth sentence of paragraph 7(c) of the Executive Stock Agreement is
     hereby amended and restated to read in its entirety as follows:

          "Noncompete Compensation" shall consist of 100% of the
          salary that Executive received under paragraph 5(d) above as
          compensation from the Company and its Subsidiaries
          immediately prior to Termination (Executive's "Previous
          Salary") together with the continuation of the medical
          benefits that the Company provided to Executive immediately
          prior to Termination (Executive's "Previous Benefits");
          provided that if at any time during the Noncompetition
          --------
          Period Executive obtains other employment (i) with
          comparable medical benefits to Executive's Previous
          Benefits, Executive's Noncompete Compensation shall during
          the period of such employment not include the continued
          provision of medical benefits, and (ii) with a salary
          exceeding 100% of Executive's Previous Salary, Executive's

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          Noncompete Compensation shall during the period of such
          employment be reduced (but not below zero) by the amount of
          such excess."

     2.   Consent.  For all purposes of the Stock Purchase Agreement (including,
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without limitation, Sections 4C(xii), 4E and 4G thereof), each of the
Stockholders consents to the amendment set forth in paragraph 1 of this
Amendment.

     3.   Counterparts.  This Amendment may be executed in multiple
          ------------                                             
counterparts, each of which shall be an original and all of which taken together
shall constitute one and the same agreement.

     4.   Descriptive Headings.  The descriptive headings of this Amendment are
          --------------------                                                 
inserted for convenience only and do not constitute a part of this Amendment.


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     IN WITNESS WHEREOF, the parties hereto have executed this Amendment as of
the date first above written.

                              MADISON DEARBORN CAPITAL PARTNERS, L.P.           
___________________________
Brian F. Addy                 By:  Madison Dearborn Partners, L.P., its General 
                                   Partner                                      
                              By:  Madison Dearborn Partners, Inc., its General 
                                   Partner

                              By:_______________________________________________
___________________________
John R. Barnicle                   Its:_________________________________________
     
    
                              FRONTENAC VI, L.P.
___________________________                                                   
Joseph Beatty                 By: Frontenac Company, its General Partner

                              By:_______________________________________________
                                   Its:_________________________________________



                              BATTERY VENTURES III, L.P. 
___________________________                                                  
          
Robert C. Taylor, Jr.         By: Battery Partners III, L.P., its General
                                  Partner 


                              By:_______________________________________________
                                   Its:_________________________________________


                              FOCAL COMMUNICATIONS CORPORATION
                              
                              By:_______________________________________________
                                   Its:_________________________________________

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