Arterial thromboembolism is a recognized complication of systemic heparin therapy. Characteristic of the entity is arterial occlusion by platelet-fibrin thrombi with distal ischemia occurring four to twenty days after the initiation of heparin therapy, preceded by profound thrombocytopenia with platelet counts in the range of 30,000 to 40,000 per cubic millimeter. The clinically apparent occlusion may be preceded by gastrointestinal and musculoskeletal symptoms that appear to be ischemic in origin, and might serve to warn the clinician of these complications. Previous reports of these phenomena as well as recent studies of the effect of heparin are reviewed. The common factor relating thromboembolism and thrombocytopenia is heparin-induced platelet aggregation. Appropriate treatment consists of discontinuation of heparin, and anticoagulation with sodium warfarin if necessary. Vascular procedures are performed as indicated.