EXHIBIT 24 POWER OF ATTORNEY KNOW ALL MEN BY THESE PRESENTS THAT the undersigned directors and officers of McKesson HBOC, Inc., a Delaware corporation (the "Company"), do hereby constitute and appoint Ivan D. Meyerson and Kristina Veaco his or her true and lawful attorney and agent, each with full power and authority (acting alone and without the other) to execute in the name and on behalf of the undersigned as such Director and/or Officer, under the Securities Act of 1934, as amended, an annual report on Form 10-K, and thereafter to execute and file any and all amendments to such Form, whether filed prior or subsequent to the time such Form becomes effective. The undersigned hereby grants unto such attorneys and agents, and each of them, full power of substitution and revocation in the premises and hereby ratifies and confirms all that such attorneys and agents may do or cause to be done by virtue of these presents. Signature Capacity --------- -------- /s/ Alan Seelenfreund Chairman of the Board ------------------------------- Alan Seelenfreund /s/ John H. Hammergren Co-President, Co-Chief Executive ------------------------------- Officer Elect and Director John H. Hammergren (Principal Executive Officer) /s/ David L. Mahoney Co-President, Co-Chief Executive ------------------------------- Officer Elect and Director David L. Mahoney (Principal Executive Officer) /s/ Heidi E. Yodowitz Senior Vice President, Controller ------------------------------- and Acting Chief Financial Heidi E. Yodowitz Officer (Principal Financial and Accounting Officer) /s/ Alfred C. Eckert III Director ------------------------------- Alfred C. Eckert III /s/ Tully M. Friedman Director ------------------------------- Tully M. Friedman /s/ Alton F. Irby III Director ------------------------------- Alton F. Irby III /s/ M. Christine Jacobs Director ------------------------------- M. Christine Jacobs /s/ Gerald E. Mayo Director ------------------------------- Gerald E. Mayo Director ------------------------------- Charles W. McCall /s/ James V. Napier Director ------------------------------- James V. Napier /s/ David S. Pottruck Director ------------------------------- David S. Pottruck Signature Capacity --------- -------- Director ------------------------------ Mark A. Pulido /s/ Carl E. Reichardt Director ------------------------------ Carl E. Reichardt /s/ Jane E. Shaw Director ------------------------------ Jane E. Shaw Dated: July 13, 1999