We evaluated platelet aggregation in vitro in blood samples drawn simultaneously from aorta and coronary sinus. Platelet aggregation was significantly lower in the coronary venous blood than in the aortic blood in patients with coronary artery disease. Lower platelet counts were also observed in coronary venous blood. No such differences were seen in subjects with normal coronary arteries. Oral administration of aspirin eliminated the differences in platelet aggregation and counts across the myocardial vascular bed. These observations suggest that platelet sequestration in the myocardial vasculature may be related to the presence of disease in the coronary arteries.