A comparative epidemiologic investigations was conducted among Japanese immigrants and their offspring living in Hawaii and the Los Angeles area, and among Japanese living in Hiroshima. All subjects received an oral glucose tolerance test and the diagnosis of diabetes was made on the basis of WHO criteria. In the subjects aged 40 years or older, the age, sex, and obesity-specific prevalence of diabetes was 2-3 times higher among Japanese living in America than those in Hiroshima. The fasting and post-glucose load serum immunoreactive insulin concentrations were higher in the Japanese-Americans compared to the Hiroshima inhabitants with the same degree of obesity and glucose tolerance. We suggest that a westernized lifestyle induces peripheral insulin resistance and promotes the development of diabetes among Japanese.