Population-based registries of insulin-dependent diabetes mellitus (IDDM) worldwide have reached the critical mass needed to investigate global patterns of the disease. International collaboration of 24 registries from 15 countries resulted in the first set of standardized incidence measures among divergent areas and ethnic groups. The average annual age-adjusted incidence under age 15 yr ranged from 1.7/100,000 person-yr in Hokkaido, Japan, to 29.5/100,000 person-yr in Finland during the years 1978-1980. The geographic differences in IDDM in childhood are rarely seen among chronic diseases. It appears that the risk for IDDM is determined by factor(s) correlated to the average yearly temperature of environment and to the ethnicity of the population at risk.