The Ob protein leptin has been shown to be closely correlated with measures of body fat in humans and animals. Studies have suggested that there are both gender and ethnic differences in serum leptin concentrations, even after controlling for total and relative body fat and body mass index. We hypothesized that gender and ethnic differences in serum leptin concentrations are due to differences in both body composition and body fat distribution. We measured fasting serum leptin concentration, body composition (fat mass and fat-free mass by dual energy x-ray absorptiometry), and body fat distribution (intraabdominal and sc abdominal adipose tissue by computed tomography) in 74 prepubertal boys and girls (43 African-Americans and 31 Caucasians). Our results showed that gender differences in serum leptin concentrations could not be fully explained by differences in body mass index, total fat mass, or relative body composition. However, when serum leptin concentrations were adjusted for differences in relative body composition (fat mass and fat-free mass) and body fat distribution (so and intraabdominal adipose tissue), gender no longer had an independent effect on the serum leptin concentration. Serum leptin concentrations were not influenced by ethnicity. Thus, when comparing group differences in serum leptin concentrations, it is necessary to adequately control for group differences in body composition and fat distribution.