A 9 1/2-year-old Taiwanese boy with Prader-Willi syndrome had the following characteristics: difficulties with sucking, feeding and hypotonia during infancy, a dysmorphic face (triangular mouth, high arched palate, almond-shaped eyes and large head circumference with a relatively narrow bifrontal diameter), borderline intelligence, hypogonadism, hyperphagia, skin picking and truncal obesity. The boy experienced two hypersomnia episodes, at age 8 and 9 years, with both episodes lasting for 10 days. During the two episodes, he was found to have an exacerbated case of hyperphagia, pica, poor emotional control, stereotyped speech and agitated behavior upon awakening. After each episode, the boy had complete remission. Our findings show that the two episodes are compatible with Kleine-Levin syndrome. The relationship between the two syndromes, the Prader-Willi syndrome and the Kleine-Levin syndrome, deserves further study.