The frequency and type of acute lesions in the four major (right, left main, left anterior descending, left circumflex) epicardial coronary arteries were examined at necropsy in 14 patients with unstable angina pectoris, 21 patients with sudden coronary death and 32 patients with a fatal first acute myocardial infarction. None of the 67 patients had a grossly visible left ventricular scar (healed myocardial infarct) and only the group with acute myocardial infarction had left ventricular myocardial necrosis. Although the frequency of intraluminal thrombus was similar in patients with unstable angina (29%) and sudden death (29%) and significantly lower than in those with acute infarction (69%) (p = 0.02), the thrombus in the patients with unstable angina and sudden death consisted almost entirely of platelets and was nonocclusive, whereas the thrombus in the group with acute infarction consisted almost entirely of fibrin and was occlusive. The frequency of plaque rupture was insignificantly different in the groups with unstable angina (36%) and sudden death (19%), and was significantly lower than in the group with acute infarction (75%) (p = 0.02). The frequency of plaque hemorrhage was insignificantly different in the groups with unstable angina (64%) and sudden death (38%) and was significantly lower than in the group with acute infarction (90%) (p = 0.04).(ABSTRACT TRUNCATED AT 250 WORDS)