We report semi-quantitative histological data on coronary arteries, collected at autopsy from Japanese of both sexes (Group I) who had had unstable anginal attacks and transient ST elevation or depression in ECG within 1 month before death. The cause of death in all cases was acute myocardial infarction or coronary sudden death. A control group consisted of 28 autopsied patients (Group II) who had died of causes other than heart disease and who had been free from anginal attacks. The frequency of 51-75% and 76-100% luminal narrowing in the coronary arteries in Group I was statistically higher than that in Group II (P less than 0.01). Subendothelial infiltration of monocytes/macrophages with edematous change was most evident in Group I in all segments of the coronary artery, particularly in the proximal portions of the three main branches, regardless of mural or occlusive thrombotic sites of the coronary artery. The subendothelial infiltration of monocytes/macrophages, in terms of luminal narrowing, was the most frequent in the portions with 0-50% luminal narrowing, followed by portions with 51-75% narrowing. The subendothelial infiltration of mononuclear cells with edematous change, observed mostly in the proximal portions of three main branches of the coronary artery in Group I, was attributed to increased subendothelial permeability and endothelial damage caused by coronary vasospasm of recent occurrence. We propose that repeated vasospasm may lead to further progression of coronary atherosclerosis.