After almost a decade of relatively few reported cases, a major resurgence of measles occurred in the United States in 1989-1990. The increase primarily involved unvaccinated racial and ethnic minority children less than five years of age residing in inner-city areas. Outbreaks of measles among vaccinated school-aged children continued to occur but had less impact than outbreaks among preschool-aged children. Efforts to prevent measles must be aimed at improving age-specific measles vaccination coverage among preschool-aged children, and implementation of a two-dose measles strategy among school-aged children.