To estimate population immunity, we examined measles immunity among residents of the United States in 1999 from serological and vaccine coverage surveys. For persons aged >or=20 years, serological data from the third National Health and Nutrition Examination Survey (1988-1994) were used. For persons <20 years of age, immunity was estimated from results of the National Immunization Survey (1994-1998), state surveys of school entrants (1990-2000), and vaccine coverage surveys of adolescents (1997). To estimate immunity from vaccine coverage data, 95% vaccine efficacy was used for recipients of a single dose at >or=12 years of age and 99% vaccine efficacy was used for those with failure of a first dose who were revaccinated. Overall, calculated population immunity was found to be 93%. Although there was not much variation in immunity by region and state, in some large urban centers immunity among preschool-aged children was as low as 86%. Overall, geographic- and age-specific estimates of a high population immunity support the epidemiological evidence that measles disease is no longer endemic in the United States.