The angiographic appearance of the coronary arteries was examined in 308 patients with acute myocardial infarction (AMI) who received high-dose intravenous thrombolytic therapy. Coronary angiography was performed on day 7 after admission to the hospital. Patients had an average of 2.4 discrete arterial narrowings or obstructions. The narrowings were proximal and related to bifurcations. Four fifths of the culprit arteries were patient; 104 (34%) had a ruptured plaque, 22 (7%) had an ulcerated plaque, and in 190 (62%) the lesions were eccentric. Patients differed from a comparable, previously studied, control series of 302 patients with chronic stable angina pectoris who had more extensive disease. They had 5.7 narrowings/patient, also located proximally and at bifurcations, but more widely distributed in the coronary tree. Patients with AMI who are suitable for thrombolysis have a unique coronary angiographic picture. The data confirm that AMI is caused by sudden rupture of a localized atheromatous plaque that initiates an obstructive thrombotic cascade.