OBJECTIVE To examine the intracountry variation of IDDM incidence in Japan and compare it with data from the British Isles and the U.S. RESEARCH DESIGN AND METHODS IDDM incidence was determined with a standardized registry approach in five geographically different areas in Japan (Hokkaido, Tokyo, Yokohama, Osaka, and Kagoshima) with a total at-risk population < 15 yr of age of 4.4 million. Data collection was completed under the guidelines of the World Health Organization Diamond project. RESULTS The incidence and patterns of disease were remarkably similar across the five environmentally different areas. Incidence rates per 100,000 were very low and almost identical from northern to southern Japan (Hokkaido, 2.07; Tokyo, 1.65; Yokohama, 1.66; Osaka, 1.78; and Kagoshima, 1.93). This remarkably low intracountry variation of 0.4/100,000 stands in sharp contrast with the 13.0/100,000 regional variation in the British Isles and the 10.6/100,000 variation in the U.S. CONCLUSIONS The results suggest that the remarkable genetic homogeneity in Japan may produce uniformity of incidence.