Estimation of body composition by measurement of tetrapolar bioelectrical resistive impedance (R) is a technique only recently validated in adults. To evaluate this technique in children, 16 girls aged 11.6 +/- 0.9 yr (mean +/- SD) and 14 boys aged 12.5 +/- 1.0 yr were assessed for fat free mass (FFM) by hydrostatic weighing, total body potassium (TBK) by whole body counting of 40K, and R by an electrical impedance plethysmograph. Significant relationships (P less than 0.001) were demonstrated between TBK and ht2/R (r = 0.92), and FFM and ht2/R (r = 0.83). The reliability coefficient for the impedance measurements was r = 0.97. The following equation describes FFM in the present group of children: FFM (kg) = 6.86 + 0.81 x (ht2/R), r = 0.83, P less than 0.001, SEE = 4.08. These data suggest that impedance measurements are valid and reliable predictors of FFM in pediatric populations.