During late March and early April 2000, four cases of meningococcal disease caused by Neisseria meningitidis serogroup W-135 were identified among U.S. pilgrims returning from the Hajj in Saudi Arabia, their close contacts, and communities. These cases occurred as part of a larger epidemic in which approximately 400 cases caused by a similar and unusual strain were identified worldwide. The Hajj, an annual pilgrimage to the major holy places of Islam, is attended by approximately two million persons from approximately 140 countries, including an estimated 15,000 from the United States.