Children in the United States are almost 20 times more likely to develop insulin-dependent diabetes mellitus (IDDM) than children in Japan. Little is known about the differences between the two countries that might account for this very large difference in risk. The current research compared the characteristics of IDDM in Japan with those of the United States (Allegheny County, Pennsylvania). Seasonality, relationship to socioeconomic status, and age at onset were similar. There was some suggestion of a sex difference. Of interest was that reported recent infections at onset were much higher in the United States. In addition, the risk to first-degree relatives in Japan appeared to be somewhat lower than in the United States, although this may have been the result of differences in ascertainment. These results are discussed in relation to potential factors that might account for the major incidence differences.