The primary objective was to investigate the influences of ethnicity and pubertal stage on the ability of body mass index (BMI) to rank adiposity in a cohort of 402 Asian, African, Hispanic, and White Americans, aged 9-25 years. BMI was calculated from measured weight and height. Body composition and pubertal stage were determined by dual energy x-ray absorptiometry and self-rating, respectively. Multiple linear regression models with BMI, ethnicity, puberty, and age as explanatory variables were used to determine if ethnicity and pubertal stage modify the ability of BMI to predict percent of body weight that is fat (%BF). Spearman's correlation coefficients of BMI with %BF were also estimated. Both ethnicity and pubertal stage interacted with BMI. The correlation between BMI and %BF (r) was much higher in females (r = 0.72, P < 0.001) than in males (r = 0.15, NS). Among females, correlations did not vary much by pubertal stage, but tended to be higher in Blacks. Among males, correlations tended to be low during mid-puberty. However, a high correlation between BMI and lean body mass (LBM) was consistently observed in mid-pubertal males across all ethnic groups. Stratification by quartile of BMI showed that the correlation was >0 (P < .05) only in the highest quartile. Thus, in an ethnically diverse cohort of 9-25 year olds, the correlation between BMI and %BF is poor in the lower quartiles of BMI. In comparison, LBM correlates better with BMI than %BF in the lower quartiles of BMI. Β© 1996 Wiley-Liss, Inc.