<Page>
EXHIBIT 3.42
AMERIPATH PENNSYLVANIA, INC.
BY-LAWS
OFFICE
1. The registered office of AmeriPath Pennsylvania, Inc., shall be
located at St. Francis Medical Center, 45th Street, Pittsburgh, Pennsylvania
15201, and may be changed from time to time by resolution of the Board of
Directors.
SHAREHOLDERS' MEETINGS
2. The annual meeting of the shareholders shall be held on such day and
at such time and place as may be designated by the Board of Directors, or, if
not so designated, as shall be agreed to by the shareholders, or if not so
designated or agreed to by the shareholders, on the 15th day of December of each
year, if not a legal holiday, then on the next business day following, at 4
o'clock P.M., local time, at the registered office of the corporation.
3. Special meetings of the shareholders shall be held at such place as
may be designated by the Board of Directors, or, if not so designated, at the
registered office of the corporation.
DIRECTORS
4. The property and business of the Corporation shall be managed by a
Board of two (2) members, as shall be determined from time to time by the
Shareholders of the Corporation.
5. The term of office of each Director shall extend from the time of his
election and qualification until the annual meeting of shareholders held next
after his election, or until his death, resignation or removal or until his
successor shall have been elected and qualified.
DIRECTORS MEETINGS
6. The regular annual meeting of the Board of Directors shall be held
immediately following and at the same place as the annual meeting of the
shareholders, and no notice shall be necessary if a majority of the full Board
shall be present, or the Board may meet at such time and place as shall be fixed
by the consent in writing of all of the Directors.
<Page>
7. Other regular meetings of the Board of Directors may be held at such
place and time as the Board of Directors may determine. Notice of such regular
meetings of the Board shall not be required to be given, except that whenever
the time or place of regular meetings shall be initially fixed or changed,
notice of such action shall be mailed promptly to each Director.
8. Special meetings of the Board of Directors may be called at any time
by the Board itself, by vote at a meeting, or by any Directors, or by the
President or Vice President, to be held at such time and at such place as the
person or persons requesting the meeting shall direct.
9. Notice of all special meetings of the Board of Directors shall be
given to each Director by being mailed on at least the third (3rd) day prior to
the date of the meeting.
OFFICERS
10. The officers of the corporation shall be chosen by the Board of
Directors for such terms and compensation as the Board of Directors may
determine and shall consist of a President, such Vice Presidents as may be
determined by the Board of Directors, a Secretary, and a Treasurer. The Board of
Directors may in the Board's discretion leave any of the foregoing offices
unfilled to the extent permitted by law. Any two or more offices may be held by
the same person. The Board of Directors may from time to time appoint such
additional officers, agents employees and independent contractors as the Board
shall deem advisable and prescribe then duties, conditions of employment and
compensation. Subject to the power of the Board of Directors, the President may
employ form time to time such other agents, employees an independent contractors
as the President may deem advisable and prescribe their duties, conditions of
employment and compensation.
INDEMNIFICATION OF DIRECTORS AND OFFICERS
11. The corporation shall indemnify every Director of officer, or former
Director of officer, or any person who may have served, at the Board's request,
as the Director or officers of another corporation in which it owns shares of
capital stock or of which it is a creditor, against expenses actually and
necessarily incurred by such person in connection with the defense of any
action, suit or proceeding in which such person is made a party by reason of
being or having been a Director or officer, or a Director of officer of such
other corporation, except in relation to matters as to which any such Director
or officer or former Director or officer shall be adjudged, in such action, suit
or proceeding, to be liable for negligence or misconduct in the performance of
duty. Such indemnification shall not be deemed exclusive of any other rights to
which those indemnified may be entitled under any By-Law, agreement, vote of
shareholders or otherwise.
-2-
<Page>
SHARES OF CAPITAL STOCK
12. The share certificates issued by the corporation shall be signed by
the President or the Vice President and by the Secretary or Treasurer or the
Assistant Secretary and sealed with the corporate seal of the corporation, and
if a Registrar has been appointed by the Board, such certificates shall be
countersigned by the Registrar.
13. The transfer of shares shall be made on the books of the corporation
only when authorized by the person named in the certificate and upon surrender
of the certificate therefor.
14. New certificates for shares of stock may be issued to replace
certificates lost, stolen, destroyed or mutilated, upon such terms and
conditions, including proof of loss or destruction and the giving of a
satisfactory bond or indemnity, as the Board of Directors may determine.
15. The corporation shall be entitled to treat the owner of record of any
share or shares of stock as the owner in fact thereof, and accordingly shall not
be bound to recognize any equitable or other claim to or interest in such share
on the part of any other person, whether or not it shall have express or other
notice thereof, except as expressly otherwise provided by the laws of
Pennsylvania.
FISCAL YEAR
16. The fiscal year shall end on Aril 30 of each year, or on such other
date as shall be fixed by resolution of the Board of Directors.
FINANCIAL REPORTS
17. The Board of Directors shall have discretion to determine whether
financial reports shall be sent to the shareholders, what such reports shall
contain, and whether such reports shall be audited or accompanied by the report
of an independent or certified public accountant.
AMENDMENTS
18. These Bylaws may be altered, amended and repealed, and new Bylaws may
be adopted, by a majority vote of the members of the Board of Directors, subject
always to the power of the shareholders to change such action.
-3-
<Page>
AMERIPATH PENNSYLVANIA, INC.
AMENDMENT TO THE BYLAWS
The Board of Directors of AmeriPath Pennsylvania, Inc. (the "Corporation")
adopted a resolution on November 29, 2001 that amended the Bylaws of the
Corporation as follows:
Paragraph 4 is deleted in its entirety and replaced with a new paragraph 4
to read as follows:
"4. The property and business of the Corporation shall be managed by a
Board of Directors. The number of directors of the Corporation shall not be
fewer than one nor more than five, the exact number to be fixed by
resolution of the shareholders or of the Board of Directors from time to
time."