1 EXHIBIT 10(a) LETTER AGREEMENT BY AND BETWEEN THE MPK STOCK OPTION PLAN AND MARY C. GERLITS DATED OCTOBER 15, 1999.

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                                                                EXHIBIT 10(a)


LETTER AGREEMENT BY AND BETWEEN THE MPK STOCK OPTION PLAN AND XXXX X.
XXXXXXX DATED OCTOBER 15, 1999.

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                                                                 EXHIBIT 10(a)
                                XXXXXXX X. XXXXXX
                           000 XXXXX XXXXXXXXX XXXXXX
                          XXXXXX XXXXX, XXXXXXXX 00000


                                October 15, 1999


Xx. Xxxx X. Xxxxxxx
0000 Xxxxxxxx Xxxxx
Xxxxxxxx, Xxxxxxxx  00000


Dear Xxxx:

     This letter is intended to memorialize our understanding and agreement
concerning the following matters discussed in our July 23, 1998 Letter Agreement
(the "1998 Letter Agreement") related to the MPK Stock Option Plan (the "Plan").
Neither this letter nor the Letter Agreement shall be deemed to amend the Plan,
but instead have been entered into to clarify the parties' rights and intentions
related to subsequent disposition of the underlying shares of CDW Computer
Centers, Inc. (the "Company") and matters related thereto.

1.   Pursuant to your request, Section 6 of the 1998 Letter Agreement shall
     be amended, as it relates to you, to continue your right (subject to all
     related obligations) to sell 25,252 Shares under the Registration Statement
     (as defined therein) (those being the shares which, pursuant to the 1998
     Letter Agreement, were saleable by you thereunder as of April 1, 1999)
     despite the termination of your employment with the Company. As soon as
     reasonably practicable after the closing of the exercise of your remaining
     options, we will take such actions as are reasonably necessary to implement
     the terms of this Paragraph 1, which in any event shall be within 10 days
     after such closing.

2.   In order to facilitate the sale of such shares, we will remove any legend
     on the certificates evidencing such 25,252 Shares (provided you agree to
     hold those certificates pending sale and delivery in accordance with the
     terms of the Registration Statement).


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                                                                  Exhibit 10(a)

Xx. Xxxx X. Xxxxxxx
October 15, 1999
Page 2


     Please acknowledge, where indicated below, your agreement to the clarifying
amendments to the 1998 Letter Agreement. All other provisions of the 1998 Letter
Agreements shall remain in full force and effect without modification and you
shall remain bound thereby.

                                            Very truly yours,

                                            /s/ XXXXXXX X. XXXXXX

                                            Xxxxxxx X. Xxxxxx

AGREED AND ACKNOWLEDGED:


/s/ XXXX X. XXXXXXX
--------------------------------
Xxxx X. Xxxxxxx