Beginning in October, 1990, a large measles outbreak involving predominantly unvaccinated preschool age children occurred in Philadelphia. By June, 1991, 938 measles cases had been reported to the Philadelphia Health Department. In addition to these cases, 486 cases and 6 measles-associated deaths occurred between November 4, 1990, and March 24, 1991, among members of 2 Philadelphia church groups that do not accept vaccination. We identified measles cases and collected information on symptoms and potential risk factors for complications. Telephone interviews were conducted to collect demographic and clinical information on measles cases in church member households. We identified 486 measles cases among 892 mostly unvaccinated church members. Age-specific attack rates were highest among children 1 to 4 years of age (94%) and 5 to 14 years of age (91%). Five (83%) of the 6 deaths occurred in females, 3 of whom had underlying illnesses. The overall case-fatality rate was 1.2%. The case-fatality rate was 2% for females, 0.4% for males (P = 0.22), 1.7% for primary cases and 0.7% for secondary household cases (P = 0.67). Only one of the children who died had received medical care. Measles spread rapidly in this group, sparing few susceptible individuals. Lack of medical care and underlying disease appear to have contributed to the high case-fatality rate in the church communities.