Total body water was measured in 26 children and adolescents using the stable isotope H2O18. Body resistance was measured using a tetrapolar technique with a constant 50Khz, 800 microA alternating current. Total body water was highly correlated (r = 0.97; P less than 0.001) with height2/body resistance. Measurements of body resistance are non-invasive, rapid and readily acceptable to children. For these reasons the measurement of body resistance requires further investigation, including cross validation studies and is a potentially valuable technique for assessing body composition in the paediatric population.