A randomized, controlled study of 54 children (age, 4-16 years) with Prader-Willi syndrome was conducted to assess the potential beneficial effects of growth hormone (GH) treatment. After observation for 6 months, the children were randomized to receive GH at a dose of 3 IU/m2/day (1 mg/m2/day) (n = 35) or no intervention (n = 19). The effects of GH treatment on linear growth, body composition, muscle strength, pulmonary function and resting energy expenditure were assessed. The levels of GH secreted in response to clonidine stimulation were universally low, and mean (+/- SD) insulin-like growth factor I SDS was -1.2 +/- 0.8 pretreatment. In children treated for 1 year, mean height velocity SDS significantly increased from -1.0 +/- 2.5 to 4.6 +/- 2.9 (p < 0.0001), mean percentage body fat decreased from 46.3 +/- 8.4% to 38.4 +/- 10.7% (p < 0.001), mean lean body mass increased from 20.5 +/- 6.3 kg to 25.6 +/- 4.3 kg (p < 0.01) and respiratory muscle function and physical strength improved. Mean respiratory quotients significantly decreased from 0.81 to 0.77 (p < 0.001); however, resting energy expenditure did not change. Therefore, GH therapy appears to reduce some of the physical disabilities experienced by children with Prader-Willi syndrome.