Body mass index is widely used as a measure of adiposity in adults, but its use in children and adolescents is controversial. We assessed body mass index as a measure of adiposity in children and adolescents between the ages of 5 and 20 yr examined as part of the NIH survey of health in the Pima Indian population. Body mass index (measured in 985 subjects and analyzed in 3 age groups: 5-9, 10-14, and 15-19 yr, in both sexes) was compared cross-sectionally to percent fat and fat mass derived from dual energy x-ray absorptiometry and to fasting and 2-h plasma glucose, systolic and diastolic blood pressures, cholesterol, high density lipoprotein cholesterol, fasting insulin, and triglycerides. Body mass index was strongly correlated in all age groups to both percent fat (r = 0.83-0.94; for each group, P < 0.0001) and fat mass (r = 0.96-0.98; P < 0.0001). The relationship of body mass index to percent fat was different in males and females; differences were more marked in older age groups. Body mass index, percent fat, and fat mass showed similar degrees of correlation to metabolic measures in childhood. Body mass index is strongly associated with measures of adiposity derived from dual energy x-ray absorptiometry. Both measures show similar associations with cardiovascular risk factors in Pima Indian children.