The body composition of a total of 90 children (aged 5-18) were subjected to multifrequency impedance measurement with a Human IM Scan impedance analyser. We compared data of 30 overweight children (17 boys, 13 girls; mean age 11.47 yrs, range: 4-17 yrs), 30 children with cystic fibrosis (12 boys, 18 girls; mean age 12.8 yrs, range: 3-24 yrs) and 30 healthy children (12 boys, 18 girls; mean age 13.4 yrs, range 7-18 yrs). The percentage of fat mass was found to be inversely proportional to total body water. Extracellular water expressed as a percentage of the total body water was significantly higher in overweight children than in the other two groups: (p < 0.01) between children with cystic fibrosis and overweight children, (p < 0.05) between healthy children and overweight children. Extracellular water expressed as a percentage of total body water was lower in children with cystic fibrosis than in healthy children (p < 0.05). The ratios of extracellular mass and body cell mass to body weight were significantly lower in overweight children than in healthy children (p < 0.01) or in children with cystic fibrosis (p < 0.01). The ratio of exchangeable sodium and potassium was lower in all three groups than the normal value given for adults (> 0.87), but in overweight children it was significantly higher than in the other two groups; (p < 0.01) between children with cystic fibrosis and overweight children, (p < 0.05) between healthy children and overweight children. Multifrequency impedance measurement is reliable, fast, non-invasive, easy to carry out, therefore we consider it very useful in determining the body composition of children in various pathological conditions.