OBJECTIVE: To describe associations between sexual maturation and body composition in a sample of African-American girls who were participants in phase 1 pilot interventions of the Girls Health Enrichment Multisite Studies. RESEARCH METHODS AND PROCEDURES: Stature, weight, and waist circumference were measured. Pubic hair and breast development were assessed, and body composition was measured by DXA for 147 African-American girls who were 8 to 10 years of age from three field centers. Participants had BMI > or =25th percentile for age (one site) or BMI > or =50th percentile for age. RESULTS: Girls Health Enrichment Multisite Studies girls had greater BMI, fat mass, and percentage body fat than national norms and relatively earlier initiation of breast development and pubic hair. Increasing stages of breast development, but not stages of pubic hair, were related to increased stature, waist circumference, BMI, lean mass, fat mass, and percentage of body fat. Pubescent girls (breast stage > or = 2) were greater than six times as likely to be classified as at risk of overweight (BMI > or = 85th percentile) and greater than eight times as likely to be classified as overweight (BMI > or = 95th percentile) as prepubescent counterparts. Adjusted odds ratios for advanced breast development [breast stage > or = 2 (8 years) or > or = 3 (9 and 10 years)] were 3.6 for risk of overweight and for overweight compared to girls with average or less than average breast development. DISCUSSION: Sexual maturation is important to consider in understanding the classification of overweight and the development of obesity during adolescence. Breast development and pubic hair development should be considered separately for their associations with growth and body composition.