Prader-Willi syndrome (PWS) is characterized by short stature, muscular hypotonia, cognitive dysfunction, and hyperphagia usually leading to severe obesity. Patients with PWS share similarities with growth hormone deficiency (GHD). Few studies have dealt with growth hormone (GH) treatment in PWS adults. The purpose of the Scandinavian study was to evaluate the effects of GH on body composition, lipid and glucose metabolism, physical performance and safety parameters in adults with PWS. Twenty-five women and 21 men with PWS were randomized to treatment with GH or placebo during 1 year followed by 2 years of open labeled GH treatment. At baseline 1/3 had normal BMI, six patients severe GHD, ten impaired glucose tolerance and seven diabetes. At 1 year insulin-like growth factor I (IGF-I) SDS had increased by 1.51 (P < 0.001) and body composition improved in the GH treated group. Visceral fat decreased by 22.9 ml (P = 0.004), abdominal subcutaneous fat by 70.9 ml (P = 0.003) and thigh fat by 21.3 ml (P = 0.013), whereas thigh muscle increased 6.0 ml (P = 0.005). Lean body mass increased 2.25 kg (P = 0.005), and total fat mass decreased 4.20 kg (P < 0.001). The positive effects on body composition were maintained after 2 years of GH treatment. Peak expiratory flow increased by 12% (P < 0.001) at 2 years of GH treatment. Lipid and glucose metabolism were unchanged, however, three patients developed diabetes at 2 years of GH treatment. In conclusion GH treatment had beneficial effects on the abnormal body composition without serious adverse events making it a logic treatment option in adults with PWS.