Background: The number of overweight children has been rapidly increasing, although its prevalence varies by age, sex, ethnicity, and socioeconomic (SES) status. Methods: Height and weight assessments were used to calculate body mass index (BMI) and BMI percentile on more than 17,000 children in 1 north Florida school district's elementary and middle schools. Based on the child's BMI percentile, each child was placed into 1 of 4 groups: underweight, normal, at risk for overweight, and overweight. Logistic regression was used to test the relative contribution of sex, ethnicity, school (elementary vs middle), age, and SES (indicated by free/reduced vs full-pay lunch status) to a child's weight classification. Results: Overall, 36.2% of the children were either overweight (18.9%) or at risk for overweight (17.4%). Approximately 30% of the kindergarten children were overweight (14.0%) or at risk for overweight (15.5%). African American children were most likely to begin kindergarten overweight. The prevalence of overweight increased for all ethnic groups during the elementary school years. However, African American girls and Hispanic boys were more likely to be overweight than any other ethnic group; Asian girls were least likely to be overweight. These findings could not be readily explained by the effects of SES. Higher SES appeared to be protective but only for white and Hispanic children. Conclusions: These results confirm the increasing prevalence of overweight in US school children, especially among African American girls and Hispanic boys. (Contains 3 tables and 4 figures.)