The accuracy of total-body electrical conductivity (TOBEC) for body composition assessment was examined in 50 teen-agers (33 males and 17 females) aged 11-19 y. Body composition measures included densitometry, hydrometry, bone mineral density of the one-third distal radius, and TOBEC. Fat-free mass (FFM) was calculated by using one-, two-, and three-compartment models: densitometry, FFMd; densitometry and hydrometry, FFMdw; and densitometry, hydrometry, and bone mineral density, FFMdwb. Correlations between TOBEC variables and the three calculations of FFM were highly significant (r = 0.88-0.95; P less than 0.01). No significant differences were observed (ANOVA) between the TOBEC estimate of FFM (FFMT) and FFMd, FFMdw, and FFMdwb. Similar results were observed when the data were analyzed by gender. The TOBEC estimate of FFM was equivalent to the estimates of FFM derived from one-, two-, and three-component models. Its ease of measurement and its prediction accuracy (R2 = 0.933; SEE = 2.45 kg) in teen-agers make it a preferred technique for body composition assessment.