A total of 9,643 measles cases was reported from the United States in 1991, a 65.3% decrease from the 27,786 cases reported in 1990. The overall incidence of measles was 3.9 cases per 100,000 population. The highest age-specific incidence was among children < 12 months of age (46.9/100,000) and 1-4 years of age (19.6/100,000). Incidence rates among American Indians, Hispanic, and black children < 5 years of age were 19, 6, and 4 times that for non-Hispanic white children, respectively. More than 61% of all cases were reported from seven large outbreaks, which involved predominantly unvaccinated preschool-age children in large urban areas. Although reported measles cases decreased in 1991 compared with 1989-1990, only a sustained effort to provide age-appropriate vaccination will prevent another resurgence of measles.