['To determine the effect of growth hormone (GH) on body composition and motor development in infants and toddlers with Prader-Willi syndrome (PWS).', 'Twenty-nine subjects with PWS (4-37 months of age) were randomized to GH treatment (1mg/m 2 /day) or observation for 12 months. Percent body fat, lean body mass, and bone mineral density were measured by dual x-ray absorptiometry; energy expenditure was measured by deuterium dilution; and motor constructs of mobility (M) and stability (S) were assessed using the Toddler Infant Motor Evaluation (TIME).', 'GH-treated subjects, compared with controls, demonstrated decreased percent body fat (mean, 22.6% +/- 8.9% vs 28.5% +/- 7.9%; P < .001), increased lean body mass (mean, 9.82 +/- 1.9 kg vs 6.3 +/- 1.9 kg; P < .001), and increased height velocity Z scores (mean, 5. 0 +/- 1.8 vs 1.4 +/- 1.0; P < .001). Patients who began GH before 18 months of age showed higher mobility skill acquisition compared with controls within the same age range (mean increase in raw score, 284 +/- 105 vs 206 +/- 63; P < .05).'] Does growth hormone improve mobility and body composition in infants and toddlers with Prader-Willi syndrome?