The determination of body composition as part of the clinical and auxologic follow up of childhood growth disorders necessitates the use of a quick, portable, reliable and simple non-invasive method. The present study was undertaken to validate bioelectrical conductance, height2/resistance (Ht2/R), against isotopically determined total body water (TBW) using heavy water tracer H2[18O]. The subjects (n = 56) consisted of normal children, children with various endocrine disorders, and young adults between the ages of 8-26 years. Isotopically determined TBW and fat free mass (FFM) were highly correlated with Ht2/R (r = 0.94, p = < 0.001, and r = 0.94, p = < 0.001, respectively). In a multiple regression analysis, 96% of the variability in FFM in normal subjects could be predicted by the following equation: FFM = 0.524 Ht2/R + 0.415 Wt-0.32, while in the group of patients by FFM = 0.659 Ht2/R + 0.254 Wt + 2.851. These data suggest that bioelectrical impedance measurements give valid and reliable estimates of FFM in children and adolescents. This easy technique could be incorporated in the auxologic follow up of children on hormone therapy.