BACKGROUND: Anthropometric measurements are widely used to determine body composition, especially in children. OBJECTIVE: Our aim was to compare 2 of the simplest anthropometry-based equations available for determining nutritional status and muscularity in children and adolescents, examined in relation to other methodologically independent muscle variables. DESIGN: Midupper arm muscle area (UAMA) and fat-free mass (FFM) according to the equations of Slaughter et al (Hum Biol 1988;60:709-23), as well as separate biochemical, physical, and radiologic muscle variables, were determined cross-sectionally in 91 males and 91 females aged 6-18 y. The ability of UAMA and FFM to estimate muscularity, as measured by 24-h creatinine excretion, grip force, and peripheral quantitative computer tomography analysis of forearm muscle, was compared after dividing the study population into prepubertal and pubertal groups. RESULTS: Before puberty, correlations of all 3 muscularity variables were higher with FFM than with UAMA in both males and females. Multiple regression analyses confirmed FFM to be the predominant predictor, with partial R(2) >/= 0.68 (P < 0.001). However, in puberty, FFM did not consistently show this major influence. Only before puberty did FFM provide a significantly better fit (P < 0.05) than did UAMA for 2 of the 3 muscularity variables in each sex. CONCLUSIONS: The FFM estimate proved to be the better predictor for muscularity in healthy prepubertal children and is on a par with UAMA during puberty. FFM can be recommended as a simple anthropometric method to assess nutritional status before puberty, at least in healthy children.