Platelet thromboemboli may be important not only in the development of some forms of atherosclerosis and occlusive thrombi, but also in initiating disturbances of the microcirculation. The latter mechanism may be significant in those cardiac deaths that are not directly attributable to advanced atherosclerotic disease or occlusive thrombi in coronary arteries. In addition, it is possible that mural thrombi and atherosclerotic lesions may fragment and shower the microcirculation with emboli. Such a mechanism appears to occur in the cerebrovascular and renal circulations, and it seems unlikely that the myocardial tissue would be spared the effects of such a mechanism. Thus, in examining the relation between platelets, thrombosis, and the complications of vascular disease, it is important to emphasize that consideration must be given to platelet thromboemboli and disturbances of the microcirculation as well as to occlusive coronary artery thrombi.