SUMMARY We quantitatively analyzed the degree and extent of coronary arterial narrowing by atherosclerotic plaques in the entire length of each of the four major coronary arteries in 27 necropsy patients with transmural acute myocardial infarction (AMI) and compared the findings with those in 22 control subjects. Of the 1403 5-mm segments examined in the 27 AMI patients, 484 (34%; controls 3%) were 76-100% narrowed in cross-sectional area by atherosclerotic plaques, 528 (38%; controls 25%) were 51-75% narrowed, 319 (23%; controls 44%) were 26-50% narrowed, and only 72 segments (5%; controls 28%) were 25% narrowed. 26 of the 27 patients the ECG was either diagnostic or strongly suggestive of AMI. In all 27 patients, however, AMI was diagnosed clinically. Death appeared to result from cardiogenic shock unassociated with cardiac rupture in 13 patients, from uncontrollable arrhythmias or conduction disturbances in five, from rupture of the left ventricular free wall in two and of the ventricular septum in three, from acute pulmonary edema not associated with shock in two, from intracerebral hemorrhage while on heparin therapy in one and from uncertain cause in one. The interval from onset of symptoms compatible with AMI to death ranged from 12 hours to 38 days (average 8 days), and was less than 24 hours in three patients and over 20 days in two. Except for two of the three patients who died during the first 24 hours after the onset of AMI, the infarcts were easily visible on gross inspection. The acute infarcts were transmural in all patients, defined as involvement of some portion of the inner half and all or portions of the outer half of the left ventricular wall.' In 19 of the 25 patients who had easily discernible infarct margins, the infarcts were large, involving more than 50% of a longitudinal dimension (i.e., from base to apex of left ventricle or involvement of more than one-third of the circumference of at least two of the six or seven ventricular slices cut at 1-cm intervals from apex to base parallel to the posterior atrioventricular sulcus). In the other six patients, the infarcts were of moderate size, involving 20-35% of the circumference of at least two ventricular slices or less than half of the longitudinal dimension of the left ventricle. No patient had a small infarct. At least two histologic sections extending from epicardium to endocardium and at least 2 cm wide from each patient were examined and the presence of coagulative type myocardial necrosis was confirmed in each by histologic examination. Patients