Prevalence and incidence of IDDM and NIDDM in children in the Tokyo Metropolitan area were estimated using data obtained from a hospital population study, a school population study, urine glucose screening and also from the central registry. The calculated prevalence of diabetes from the data of the central registry in Tokyo was 4 or 6 per 100,000 children. The number of schoolchildren with IDDM was estimated to be about 20 from the data of the school population study performed in a population of about 220,000, giving a prevalence rate of approximately 9.1 per 100,000 schoolchildren. From the data of our hospital population study, the overall annual incidence of IDDM was approximately 3.3 per 100,000 population. It is apparent that the incidence and prevalence of IDDM in Japan are significantly lower than in the United States and European countries.