Background Prader-Willi syndrome (PWS) children have impaired growth, and abnormal body composition. Previous 1-year controlled studies showed improvement of height and body composition during GH-treatment. Objective To evaluate growth, body composition and body proportions during GH-treatment in a large group of PWS children. Design/patients We performed a randomized controlled GH trial in 91 prepubertal PWS children (42 infants, 49 children, aged 3-14 years). After stratification for age, infants were randomized to GH-treatment (GH-group; 1 mg/m(2)/day; n = 20), or no treatment (control group; n = 22) for 1 year. In the second year all infants were treated with GH. After stratification for BMI, children > 3 years of age were randomized to GH-treatment (GH-group; 1 mg/m(2)/day; n = 27) or no treatment (control group; n = 22) for 2 years. Anthropometric parameters were assessed once in every 3 months. Body composition was measured by Dual Energy X-ray Absorptiometry. Results Median (interquartile range, iqr) height SDS increased during 2 years of GH in infants from -2.3 (-2.8 to -0.7) to -0.4 (-1.1-0.0) and in prepubertal children from -2.0 (-3.1 to -1.7) to -0.6 (-1.1 to -0.1). In non-GH-treated children height SDS did not increase. Head circumference completely normalized during 1 and 2 years of GH in infants and children, respectively. Body fat percentage and body proportions improved in GH-treated children, but did not completely normalize. Lean body mass SDS improved compared to the control group. Serum IGF-I increased to levels above the normal range in most GH-treated children. Conclusions Our randomized study shows that GH-treatment in PWS children significantly improves height, BMI, head circumference, body composition and body proportions. PWS children are highly sensitive to GH, suggesting that monitoring of serum IGF-I is indicated.