We describe prevalence and trends in overweight among children and adolescents (6 to 17 years old) in the US population and variation in the prevalence by sex, age, race-ethnicity, income, and educational level. Height and weight were measured in nationally representative surveys conducted between 1963 and 1994: cycles II (1963 to 1965) and III (1966 to 1970) of the National Health Examination Survey (NHES) and the National Health and Nutrition Examination Surveys (NHANES I, 1971 to 1974; NHANES II, 1976 to 1980; and NHANES III, 1988 to 1994). Overweight was defined by the age- and sex-specific 95th percentile of body mass index (BMI) from NHES II and III. BMI values between the 85th and 95th percentiles were considered an area of concern, because at this level there is increased risk for becoming overweight. Approximately 11% of children and adolescents were overweight in 1988 to 1994, and an additional 14% had a BMI between the 85th and 95th percentiles. The prevalence of overweight did not vary systematically with race-ethnicity, income, or education. Overweight prevalence increased over time, with the largest increase between NHANES II and NHANES III. Examination of the entire BMI distribution showed that the heaviest children were markedly heavier in NHANES III than in NHES, but the rest of the distribution of BMI showed little change. Data are limited for assessing the causes of the rapid change in the prevalence of overweight. The increased overweight prevalence in US children and adolescents may be one manifestation of a more general set of societal effects. Childhood overweight should be addressed from a public health perspective.