OBJECTIVE To assess the relationship between pubertal maturation and obesity in 9- and 10-year-old black and white girls. METHODS Cross-sectional analysis of cohort baseline data. METHODS A cohort of 2379 girls recruited from selected schools in Richmond, Calif., and greater Cincinnati, Ohio, and from the membership rolls of a prepaid group practice in greater Washington, D.C. RESULTS Sixty-four percent of black girls had begun pubertal maturation compared with 33% of white girls. In prepubertal girls, racial differences in height, weight, body mass index (kilograms per square meter) and skin-fold thickness were not significant. Within each race, onset of pubertal maturation was associated with greater height, weight, body mass index, and skin-fold measurements. Within 9-year-old girls who had begun pubertal maturation but not reached menarche, black girls were taller and heavier than white girls. Among pubertal but premenarcheal 10-year-old subjects, black girls were taller and heavier and had greater body mass index and subscapular skin-fold values. After analyses were adjusted for pubertal maturation stage by means of pubic hair development, 10-year-old pubertal black girls remained taller and heavier, but racial differences in body mass index and the sum of skin-fold measurements ceased to be significant. CONCLUSIONS These findings suggest that the initiation of racial differences in obesity are related, at least temporally, to pubertal maturation.