Skin necrosis similar to that induced by warfarin was seen in a patient who had never received the drug but who was vitamin K-deficient due to malnutrition and prolonged treatment with broad-spectrum antibiotics. He also had end-stage renal failure and was receiving prophylactic subcutaneous heparin therapy because of immobilization. His plasma protein C antigen level and, disproportionately, his plasma protein C functional activity were decreased. Both protein C values improved after vitamin K therapy, discontinuation of heparin, and initiation of hemodialysis. We surmise that skin necrosis occurred as a result of protein C deficiency caused by vitamin K depletion. Production of abnormal (descarboxy) protein C/protein S due to vitamin K deficiency and increased protein C inhibitory activity associated with renal failure and/or heparin administration may have contributed to the clinical picture. This rare but serious complication of a relatively common disorder, viz., vitamin K deficiency, reinforces the importance of vitamin K supplementation in malnourished patients who receive long-term antibiotic maintenance therapy.