An obese female patient aged 47 with a personal and familial history of recurrent venous thrombosis, who developed a coumarin-induced skin necrosis is presented. Laboratory investigations, performed three months after the acute event and in absence of coumarin therapy, emphasized a decreased anticoagulant activity of her plasma protein C (50% of the normal). These results as well as the high incidence of thrombotic disease in her relatives point to a familial heterozygous protein C deficiency. The antithrombotic role of the protein C system and the mechanism of coumarin induced necrosis of the skin are briefly discussed.