The overall annual IDDM incidence rates by area in Japan for 1985-1989 for children 0-14 years of age at diagnosis were from 1.65 to 2.07 per 100,000. The incidence in males and females did not show any temporal trends during the period between 1980 and 1989. The prevalence of IDDM was about 1 per 10,000. The clinical features at diagnosis among Japanese IDDM children identified during the 2-year period between 1979 and 1980 were as follows. Fourteen percent of the cases were in coma and 12% of the cases were asymptomatic at diagnosis. There is a suggestion that slow onset IDDM is often seen in Japan. In these children, the decline of serum CPR levels and the prevalence of ICA (islet cell antibodies) over the course of diabetes was slower than in those with an abrupt onset classical IDDM. During the period from 1975 through 1990 the incidence rates of NIDDM in school children showed as much as an approximate 1.5-fold increase along with a similar increase in the prevalence of obesity. About eighty percent of these NIDDM children were obese. A predominance of female children developing diabetes was seen in both type of diabetes, IDDM and NIDDM, in Japan. Non-obese type NIDDM in children was more common in females than in males. It is interesting to note that the mean height of Japanese children with IDDM was not different from the national average, but children with NIDDM were significantly taller than the national average.