We have compared the growth and the body composition in children with Prader-Willi syndrome (PWS) with and without growth hormone treatment (recombinant GH 0.1 IU/kg/day) after a 1-y period. Twenty-nine prepubertal children with PWS, with mean body mass index (BMI) SDS of 2.2, and 10 (control) healthy obese children with mean BMI SDS of 5.6, underwent 24-h frequent blood sampling. Both PWS and control obese children had low and similar GH levels (0.7 microg/l +/- 0.4SD). Serum IGF-I levels, however, were significantly lower in children with PWS (-1.5SDS +/- 0.8SD vs -0.2SDS +/- 0.8SD). The 29 PWS children were randomized into 2 groups of 15 and 14 subjects for GH treatment and no treatment, respectively. Height velocity increased from -1.9SDS to + 6.0SDS in the treated group (p < 0.001) and decreased from -0.1SDS to -1.4SDS in the control PWS group during the study year. BMI decreased significantly for the treated group (+3.0SDS to +2.0SDS). Relative fat mass decreased significantly, while fat-free mass increased (p < 0.001) for the treated group. No significant changes were noticed in body composition in the control PWS group. In conclusion, the low spontaneous 24-h GH secretion, regardless of body weight, and the exceptional response to growth hormone treatment together with the finding of low IGF-I levels suggest that growth hormone deficiency is a common feature of PWS, as a result of hypothalamic dysfunction. Treatment with growth hormone is beneficial for the majority of PWS children.