Body composition and leptin were studied in 13 young, still underweight and 10 older overweight children with Prader-Labhart-Willi syndrome. Not only the older overweight children but also the young underweight children had elevated skinfold standard deviation scores for body mass index and elevated body mass index adjusted leptin levels, suggesting relatively increased body fat despite underweight. Our data indicate that body composition in Prader-Labhart-Willi syndrome is disturbed already in infancy, long before the onset of obesity. Leptin production appears to be intact.