We studied all severely retarded children in a defined area of Japan. The area is a small and rural prefecture with the total population of 880,000. The population of children aged 6-14 years was 102,000. The number of severely retarded children (those who neither walk nor have IQs over 35) was 50. The prevalence rate was 0.49/1,000 and its 95% confidence interval was 0.36-0.65. The rate was significantly lower than that has been reported 20 years ago from another prefecture in Japan. Among the cases, children with very severe retardation (those who are bedridden with their IQs less than 20) accounted for 66% and children who were institutionalized for permanent care accounted for 46%. Association of several disabilities was as follows: cerebral palsy 62%, epilepsy 82%, decreased visual acuity 42%, respiratory distress 16%, and feeding difficulty 32%. The causes of prenatal origin accounted for about two-thirds of the total causes. We concluded that the prevalence rate of severely retarded children is becoming lower in Japan and that the prenatal factors play an important role in the pathogenesis of severe retardation.