A case of skin necrosis caused by subcutaneously administered heparin is reported. A 76-year-old woman received subcutaneous injections of porcine sodium heparin twice a day to prevent deep vein thrombosis. Nineteen days after heparin therapy began, black necrotic areas were noted on her abdomen, and heparin injections were discontinued. The patient received small amounts of heparin intravenously for three additionally days without apparent complications. Proposed mechanisms for heparin-induced skin necrosis include allergic vasculitis and localized platelet aggregation with intravascular thrombosis. Heparin therapy should be stopped if necrosis develops. Intravenous administration of heparin to sensitive patients may be followed by life-threatening reactions. Necrotic areas may heal spontaneously, but often require debridement and skin grafting. Various agents, including steroids, may be useful in preventing the development of necrotic lesions.