The pathology of five aneurysms resected from four patients with Kawasaki disease was examined to elucidate the mechanisms of regression. (1) Marked intimal thickening was present in all five aneurysms. (2) Two patients treated with aspirin early in their course showed well-regenerated endothelium and marked thickening of the intima without massive thrombus; the thickened intima was rich in smooth muscle cells. These aneurysms maintained an adequate lumen of similar diameter to normal arteries, and some regressed angiographically. (3) Two patients untreated with aspirin in the acute phase had intimal thickening associated with massive thrombus formation and calcification. The pathologic appearances were similar to those of early atherosclerosis. One patient died suddenly of myocardial infarction. We conclude that the angiographically demonstrated phenomenon of aneurysm regression may result from intimal thickening mainly caused by the proliferation of smooth muscle cells not associated with massive thrombus. The thickened intima associated with massive thrombus may cause ischemic heart disease and simulate atherosclerosis. It is possible that the administration of aspirin may prevent massive thrombus formation. We hypothesize that Kawasaki disease may be an etiologic factor in some cases of early atypical coronary atherosclerosis.