Prader-Willi syndrome (PWS) is a genetically determined neurodevelopmental disorder characterized by mental retardation and distinct physical, behavioral, and psychiatric features. Based on parents' questionnaires, we examined the prevalence of behavioral and psychiatric disorders of 165 persons with PWS aged 2-31 years in Japan. The data were analyzed comparing four different age groups with PWS: group 1, 2-5 years (n=34); group 2, 6-11 years (n=57); group 3, 12-17 years (n=45); and group 4, 18-31 years (n=29). Further, we compared the results of our PWS group 4 with those of 42 age-, gender-, and intelligence level-matched intellectual disability (ID) individuals without PWS. Our results showed that repetitive speech and stubbornness were prominent from early childhood and other behavioral problems such as hyperphagia, stealing food, temper tantrums, lying, and emotional lability tended to be more frequent with age among persons with PWS. Moreover, young adults with PWS have significantly higher rates of behavioral and psychiatric disorders than IDs without PWS, such as stubbornness, hyperphagia, temper tantrums, self-injurious behavior (skin picking), hypersomnia, inactivity, and delusion. Degree of obesity was not necessarily related to behavioral and psychiatric features associated with PWS. Our findings revealed that persons with PWS are more vulnerable to behavioral and psychiatric disorders particularly in young adulthood compared to those with ID from other etiologies in Japan.