Prader-Labhardt-Willi syndrome (PLWS) is a model to study GH secretion, body composition and consequences of GH therapy. Twenty-seven patients were studied by dual-energy X-ray absorptiometry (DXA) and were each compared to two age- and sex-matched controls (obese and normal weight). Fat mass (FM) was significantly greater in PLWS than in patients with simple obesity; lean body mass (LM) and bone mineral content (BMC) were significantly lower compared to both controls. The peculiar body composition of PLWS patients seems to be similar to that found in GH deficiency. In six PLWS children treated with GH, LM increased after 6 months (p<0.02) up to 12 months (p<0.03); FM decreased in 5/6 patients. Obese adult PLWS patients treated with GH for 6 months showed a reduction in adiposity; LM increased significantly only in the leg compartment. Abdominal CT scan did not show a significant reduction of intrabdominal fat area. In conclusion, GH therapy might improve final stature and exert a positive influence on body composition in patients with PLWS.