Among children and adolescents in the United States, both obesity and type 2 diabetes are more prevalent in black and Hispanic subjects. This review summarizes research to date regarding ethnic differences in body composition, body fat distribution, metabolism, pubertal timing, and endocrine status that may contribute to observed disparities in obesity and associated diseases. Research indicates that black children and adolescents have greater bone mineral content and bone mineral density than their white, Hispanic, and Asian counterparts. Similar results have been found at and during puberty. Cross-sectional studies have yielded discrepant results regarding ethnic differences in soft tissue mass. It appears that white children have greater intra-abdominal adipose tissue than black children. Studies on differences in adiposity between black and white subjects report conflicting results. Data do indicate greater fat mass among Hispanic children and adolescents versus white and black subjects. In most studies undertaken, circulating insulin-like growth factor I levels are elevated in black versus white and Hispanic prepubertal children. However, variation in circulating insulin-like growth factor I (free or bound) does not explain ethnic differences in body composition. Insulin, particularly after a glucose challenge, is higher among black versus white subjects, and may be responsible for lower rates of lipolysis and perhaps greater adipose tissue accrual among black subjects. Limited data suggest that estradiol is higher in black versus white subjects. Numerous data indicate that resting energy expenditure is lower among black versus white subjects. Data regarding physical activity are less clear but indicate that at least some populations of black children are less physically active than white children. Whether ethnic differences in resting or physical activity-related energy expenditure contribute to differences in obesity or disease risk remains to be determined. © 2003 Lippincott Williams & Wilkins, Inc.