To clarify the patho-etiologic factors of Japanese myocardial infarction, a comparative pathological study of myocardial infarction in the Osaka, Akita, Wakayama, and Hokkaido districts, and an extensive histopathological study of 94 autopsy cases with acute myocardial infarct (AMI) in less than 4 weeks at Osaka were carried out. Although AMI in Akita was highly complicated by hypertension, AMI in Osaka was associated with a history of diabetes mellitus and hypercholesterolemia, especially in the young generation (under 59 years of age); hypercholesterolemia was related to the occurrence of AMI. Moreover, in spite of increases in transmural myocardial infarct (TMI) in Osaka, Hokkaido and Wakayama, Akita showed an equal ratio of TMI and subendcardial myocardial infarct. In AMI in Osaka, significant stenosis (more than 75% stenosis) of the coronary artery was of the same grade between the proximal and distal portions in the epicardial coronary artery. AMI in Akita, however, showed more severe stenosis in the proximal than the distal portion. A high incidence (88.3%) of thrombosis formation corresponding to the site of infarction was observed in AMI in Osaka. Moreover, ruptured atheromatous plaques were identified as being responsible for 62.6% of the coronary thrombosis cases, and a high incidence (70.0%) of foamy cell infiltration was disclosed. Thus, it can be concluded that ruptured atheromatous plaque is a major factor in the progression of coronary atherosclerosis and/or thrombosis, which might be due to the process of plaque softening.