A review of 494 myocardial lesions at least 3 cm. in one dimension reveal 418 (85 per cent) related to atherosclerotic coronary lesions, 55 (11 per cent) related to coronary emboli of various types, 18(3.5 per cent) without specific coronary lesions but related to clinical events associated with coronary hypoperfusion, and 3 (0.5 per cent) associated with miscellaneous coronary lesions. In 399 of 418 (96 per cent) atherosclerotic coronary lesions of all ages complete occlusion (remote of fresh) or histological evidence of lumenal recanalization was present. These coronary lesions were situated within extramural coronary artery segments one to several centimeters proximal to the myocardial lesions which were confined to the distribution of the respective partially or totally occluded coronary segments. In the atherosclerotic cornary lesions less than 2 weeks of age partially or totally occlusive thrombus was found in 67 of 69 (97 per cent) cases and a underlying plaque ulceration, erosion, or rupture was present in 64 of 69 (93 per cent) instances. These endothelial and intimal injuries were generally focal in nature, often extending over a length of only 100 to 200 mu. In no instance could it be stated with certainty that the oldest portion of the atherosclerotic ulceration-thrombus complex was younger in age than its associated myocardial lesion. On the contrary, in 10 of 69 (14 per cent) of the cases portion of the coronary thrombus, usually at the site of plaque ulceration, were histologically older than the myocardial lesion. In addition, the presence of thrombus and plaque debris admixtures further suggested the antecedent nature of the coronary lesion in relation to the myocardial lesion. Atherosclerotic coronary lesions associated with myocardial lesions of 2 to 8 weeks of age had identifiable thromboses in all instances and underlying plaque ulceration, erosions, or ruptures in 17 of 21 (80 per cent). Endothelial injuries were more difficult to assess due to the obscuring features of organizing lumenal thrombus. Interface i.e., plaque ulceration, erosions, or ruptures, were reliably detectable up to approximately on month of age. Coronary arter thromboemboli accounted for a significant percentage of myocardial lesions, were usually associated with normal or minimal coronary artery disease, and frequently involved smaller intramural coronary vessels of the heart. Organization and recanalization of thromboemboemboli tended to be rapid and complete so that in the late stages the residual intimal plaque was sometimes difficult to identify. Myocardial lesions related to clinical events associated with coronary artery hypoperfusion centric, and not confined to the distribution of a single coronary artery. They were unassociated with acute coronary lesions and histologically displayed contraction band necrosis more frequently than the embolic and atherosclerotic related lesions. An explanation was found for the overwhelming majority of myocardial lesions...