A specific search for intramyocardial platelet aggregates was made in 90 patients who died suddenly of ischemic heart disease. Platelet aggregates in small intramyocardial vessels were found in 27 (30%). There was a significant difference (p less than .05) in the incidence of platelet aggregates in patients with chest pain of recent onset (unstable angina) before death (16/36, 44.4%) and that in those without it (11/54, 20.4%). Multifocal microscopic necrosis with involvement of the full thickness of the ventricular wall, including the subpericardial zone, was significantly more common (p = less than .005) in the patients with platelet emboli (55.6% vs 12.7%). With one exception, aggregates were confined to the segment of myocardium immediately downstream of a major epicardial coronary artery containing an atheromatous plaque that had undergone fissuring and on which mural thrombus had developed. The results support the view that platelet aggregates in the myocardium represent an embolic phenomenon and are a potential cause of unstable angina. The association of myocardial necrosis with such emboli could precipitate sudden death from ventricular fibrillation.