Prader-Willi Syndrome (PWS) is a multisystemic genetic disease characterized by hypothalamic hypogonadism, mental retardation and compulsive hyperphagia associated with early and severe obesity. Complications of overweight, such as type-2 diabetes Mellitus, dyslipidemia and diffuse atheromatosis are common. We report a 15 years old morbid obese male with PWS, with a body mass index of 57.7 kg/m², refractory to weight-lowering treatments. He underwent preoperative evaluation and treatment by a multidisciplinary team, and subjected to a 95% gastrectomy, leaving a 50 ml remnant pouch and a long limb (120 cm) Y-Roux gastro-jejuno anastomosis. There were no surgical complications, oral feeding was initiated at the 5th day with an hypocaloric diet. During the first postoperative year, the patient lost 70 kg, achieving a body mass index of 30 kg/m². Surgical treatment can become a therapeutic choice for obesity in PWS patients (Rev Méd Chile 2003; 131: 427-31).