An epidemiological survey of juvenile-onset insulin dependent diabetes mellitus (0-15 years) was performed in Hokkaido, Japan, during 1973-1981. The mean annual incidence during the nine year period was 1.26 per 100,000 and showed a significant increasing trend. The prevalence of IDDM at the ages of 0-15 and 6-15 were 7.7 and 11.7 per 100,000 respectively. These figures were much lower than those previously reported in Caucasians. Peaks in incidence were seen from February to April, and a reduction in incidence from July to September. The females showed a bimodal distribution with the peak incidence at 10 to 12 years and with a smaller peak at about 7 years. The males showed a broad single peak at about 10 to 13 years.