Autopsies were conducted on Japanese men from 45 to 71 years of age in Hiroshima, Japan (191 patients) and in Honolulu, Hawaii (298 patients). They were performed according to a common protocol. In both locations the patients studied were representative of decedents from population-based cohorts with respect to age and cause of death. The degree of atherosclerosis in the coronary arteries and aorta, assessed according to the panel method, was found to be substantially more severe in men in Hawaii than in those in Japan. Areas of recent myocardial necrosis were 3.5 times more frequent in men in Honolulu than in those in Hiroshima. Large myocardial scars were 1.5 times more frequent in the men in Honolulu. It was characteristic of men in Honolulu that severe atherosclerosis and myocardial lesions appeared at younger ages than in those in Hiroshima. These data demonstrate that there is an increase in the frequency and severity of ischemic lesion in the myocardium of Japanese who have migrated to Hawaii and that this increase is the result of atherosclerosis of the extramural segments of the coronary arteries.