Prader-Willi Syndrome (PWS) is a genetic disorder characterized by hypotonia, mental retardation or learning disability, hyperphagia and compulsive eating due to hypothalamic dysfunction. Obesity is a major cause of increased morbidity and mortality among patients with PWS. Gastric restrictive surgery has been associated with partial breakdown of the staple-line in PWS. We report two patients with PWS associated with morbid obesity and obstructive sleep apnea who underwent biliopancreatic diversion (BPD). A 27-year-old male with BMI 52 kg/m(2) and a 20 year-old female with BMI 64 kg/m(2) underwent BPD. No perioperative complications were observed. After BPD, the male's BMI was 36.7 kg/m(2) at 12 months and the female's BMI was 48.4 kg/m(2) at 28 months, with excess weight loss 58% and 48%, respectively. They developed loose stools associated with eating. These patients have shown a considerable improvement in hypersomnia and respiratory difficulties. BPD proved to be an effective approach to weight loss in PWS, resulting in improvement of sleep apnea, behavior problems and quality of life.