Thrombotic complications of heparin administration were observed in eight patients during a two year period. At sites of subcutaneous heparin injection, six patients developed areas of the skin and subcutaneous necrosis. Systemic thrombotic events and thrombocytopenia were observed in two of these patients when they received intravenous heparin and in two other patients who did not have primary skin necrosis. The complications included peripheral ischemia in three patients (two requiring amputation), myocardial infarction in two, and a cerebral infarction in one. All patients were receiving heparin for at least six days before complications occurred. Seven patients received heparin of bovine origin. Heparin-induced in vitro platelet aggregation was present in all six of the eight patients tested. (It was marked in four of these patients). It is theorized that skin necrosis and the other thrombotic complications observed are the result of heparin-induced in vivo platelet aggregation followed by intravascular thrombosis. Heparin-induced skin necrosis is a rare but serious hazard encountered with prophylactic heparin regimens. If heparin-induced thrombosis is present, the further use of heparin is contraindicated in most instances.