Clinical and morphologic findings are described in a 22 year old man with prolonged thromboyctosis, and coronary and splenic arterial thrombi causing myocardial and splenic infarcts. The absence of preexistent extensive coronary atherosclerosis, the presence of thrombus in more than one epicardial artery and in multiple intramural coronary arteries, the presence of arterial thrombosis in a noncoronary artery (splenic) and the absence of another apparent cause of the arterial thromboses are evidences that the intraarterial clotting in this patient was related to the severe thrombocytosis. A reveiw of the reported cases of vascular occlusion associated with thrombocytosis indicates that thrombi have infrequently been confirmed as the mechanism of the vascular occlusion. Although the frequency of vascular thrombi in patients with thrombocytosis has not been established, it is clear that vascular thrombosis can be a consequence of thrombocytosis and, as demonstrated by the present patient, that the coronary artery may be the site of the vascular occlusion, a heretofore unconfirmed event.