The Prader-Willi syndrome (PWS) is a rare disorder characterized by infantile hypotonia, subsequent obesity with hyperphagia. Hypogonadism, cryptorchidism, delayed psychomotor development, short status, strabismus, myopia, scoliosis, kyphosis and temperature regulation abnormality are other features. The cause of this syndrome is unknown, but a disturbance in the hypothalamus has been postulated because of the various manifestation of the syndrome. The major genetic mechanism giving rise to PWS is a paternal deletion of about the same size in the 15q11-q13 region, that occurs in 70% of the cases. The principal problems related to anesthesia are those that are secondary to the patient's hypoglycemia, skeletal muscle hypotonia and obesity. An 20-month-old boy with PWS was scheduled for surgical correction of bilateral undescended testes. The trachea was intubated with the aid of succinylcholine 7.5 mg intravenous injection. Muscle relaxation was facilitated with intermittent intravenous administration of atracurium (total dose 8 mg). No prolonged effect of muscle relaxants was observed during anaesthesia. High grade fever (38-39℃) was present during anaesthesia.