At least part of the short stature in Prader-Willi syndrome may be explained by a decreased growth hormone (GH) secretory capacity, which occurs in most patients. To study the effects of exogenous GH on growth and body composition, 17 prepubertal children with Prader-Willi syndrome, with a short projected final height, were randomized to a control group (n = 9) or a treatment group (n = 8). Children in the treatment group received GH (0.15 IU/kg/day s.c.) for 1 year. One patient in the treatment group developed pseudotumour cerebri, which resolved after discontinuation of GH; this patient was omitted from further analysis. After 1 year, height velocity in the GH-treated group was significantly increased (+5.5 SD) compared with reference values for normal healthy children, whereas there was a decreased in the control group (-2.3 SD). The difference in height velocity between the treated and control groups was significant (p = 0.0012). Concentrations of both insulin-like growth factor I (IGF-I) and IGF-binding protein-3 increased significantly in the GH-treated group (p < 0.008). A gain in height was noted for chronological age (+1.07 SD) after 1 year of GH treatment. Height gain (+1.02 SD) remained unchanged when analysed in relation to bone age. No differences between the groups were found for parameters of weight and body composition. In conclusion, although GH appears to have beneficial effects on height, long-term studies are necessary before recommendations can be made concerning GH treatment in children with Prader-Willi syndrome.