In 1956, Prader and Willi first described a clinical syndrome that included severe neonatal hypotonia, hyperphagia, obesity, diabetes, hypogonadism, cryptorchidism, dental caries and mental deficiency. We have anesthetized a male patient who had Prader-Willi syndrome. He suffered for both pyoknee. General anesthesia was performed using N2O-O2-isoflurane. During induction and maintenance of anesthesia, we focused on the airway management, hypotonia, abnormal glucose metabolism, protection of aspiration and cardiovascular stabilization. Emergence of anesthesia was unremarkable. But he was expired from sepsis on the fourth postoperative day.