A rare complication of subcutaneous heparin therapy is reported in a 75-year-old man. Large erythematous tender plaque developed in the skin overlying the injection sites. In one area of the abdomen, frank necrosis occurred with subsequent eschar formation. Skin testing with five brands of heparin revealed that the patient had a delayed type of hypersensitivity to heparin. In contrast to coumarin necrosis, heparin necrosis is felt to be a thrombotic phenomenon resulting from immunologically induced platelet aggregation in heparin-sensitive patients. Thrombocytopenia is usually present. The diagnosis can be confirmed by platelet aggregometry and calls for immediate cessation of the heparin injections.