We examined 23 pairs of black and white premenopausal women to determine whether there were ethnic differences in body composition, The pairs were matched on weight and height. Each woman had measurements of total body water taken by a tritiated water dilution technique, total body nitrogen, and total body carbon by neutron-activation, mineral ash by dual x-ray absorptiometry, and body potassium by whole body counting. Differences between blacks and whites were compared with the use of both the two-compartment and four-compartment models. The two-compartment model showed that in premenopausal black and white women of similar age, heights, and weights, blacks had significantly more lean mass. The four-compartment model resulted in similar conclusions. The black women had larger protein, mineral, and water compartments and less fat than whites. It may be that body weight measurements as an indicator of obesity should be adjusted for black versus white women.