Data obtained in the third National Health and Nutrition Examination Survey (NHANES III), conducted during 1988-1994, were analyzed to determine the epidemiology of rubella seropositivity in the United States, including risk factors for low rubella seropositivity. Serological samples obtained from NHANES III study participants > or =6 years of age were tested for rubella IgG antibodies. "Rubella seropositivity" was defined as serum rubella IgG antibody level > or =10 IU by enzyme immunoassay. Overall, rubella seropositivity rates in the United States were 92% in persons aged 6-11 years, 83% in persons aged 12-19 years, 85% in persons aged 20-29 years, 89% in persons aged 30-39 years, and >or =93% in persons aged > or =40 years. The lowest rate (78%) of any United States birth cohort of the 20th century occurred among persons born from 1970-1974. Eliminating rubella and chronic rubella syndrome in the United States will require international efforts, including vaccination of preschool- and school-age children and all susceptible young adults.