Fat and muscle areas can be calculated from equations on the basis of upper arm circumference (C) and triceps skinfold thickness (TS). These equations assume a circular limb and muscle compartment and a symmetrically distributed fat rim: total upper arm area (TUA) = C2/(4 pi), upper arm muscle area (UMA) = [C - (TS x pi)2]/(4 pi), and upper arm fat area (UFA) = TUA - UMA. This traditional method underestimates the degree of adiposity. We propose that the unrolled fat rim is a rectangle whose length = C and width = TS/2. The following new indexes are based on this assumption: upper arm fat area estimate (UFE) = C x (TS/2), and upper arm muscle area estimate (UME) = TUA - UFE. To validate these equations, areas were measured with magnetic resonance imaging (MRI) in 28 children aged 9-15 y (17 control subjects and 11 obese subjects). Correlations between MRI and UFA and MRI and UFE were similar (r = 0.96 for both correlations in the control group and r = 0.84 and 0.82, respectively, in the obese group), but the areas assessed by MRI (13.8 cm2) were closer to UFE (12.4 cm2) than to UFA (11.2 cm2) in the control group as well as in the obese group (MRI = 48.7 cm2, UFE = 46.6 cm2, and UFA = 38.5 cm2). The limits of agreement between MRI and anthropometry were 5.7 +/- 5.8 cm2 for UFA and 0.6 +/- 5.0 cm2 for UFE, showing that UFA is not acceptable in most cases, whereas UFE measurements are close to MRI measurements. In conclusion, UFE and UME are simple and accurate indexes to assess body composition. French reference values are available from 1 mo to 17 y of age.