Among men of Japanese ancestry, there is a gradient in the occurrence of coronary heart disease (CHD). It is lowest in Japan, intermediate in Hawaii, and highest in California. This gradient appears not to be completely explained by differences in dietary intake, serum cholesterol, blood pressure or smoking. To test the hypothesis that social and cultural differences may account for the CHD differences between Japan and the United States, 3809 Japanese-Americans in California were classified according to the degree to which they retained a traditional Japanese culture. The most traditional group of Japanese-Americans had a CHD prevalence as low as that observed in Japan. The group that was most acculturated to Western culture had a three- to five-fold excess in CHD prevalence. This difference in CHD rate between most and least acculturated groups could not be accounted for by differences in the major coronary risk factors.