To correlate morphologic subgroups of left ventricular aneurysms with the major clinical manifestations (ventricular arrhythmias, congestive heart failure, and systemic emboli), we studied 73 patients with such aneurysms—34 at autopsy and 45 after surgical excision. Sixty-three patients had a history of congestive heart failure, 5 had systemic emboli, and 28 had recurrent ventricular tachycardia. Two types of endocardial abnormalities were noted: thick endocardial plaque composed of fibrous elastic tissue that contained little or no thrombus (type I, 30 patients); and aneurysms with little or no endocardial fibroelastosis (type II, 43 patients), some with layers of thrombus obliterating the aneurysmal cavity. Emboli only occurred in 5 patients with type II aneurysms who had mural thrombi. Recurrent ventricular tachycardia occurred in 20 patients with type I aneurysms but only in 8 with type II (p < 0.001), showing a relation between endocardial fibroelastosis and ventricular tachycardia. Structural differences in ventricular aneurysms thus appear to have pathophysiologic significance. Our findings have implications for the mechanisms of ventricular tachycardia in postinfarction patients and for its prevention by endocardial resection.