Protein C (PC), a 62,000-molecular weight vitamin K-dependent serine protease zymogen, is a natural anticoagulant that occurs in plasma at 4 mg/L. Activated PC inactivates clotting factors V and VIII and is also profibrinolytic. Activated PC is enhanced in its anticoagulant activity by protein S (PS), another vitamin K-dependent protein. Protein S is found in platelets and endothelial cells as well as in plasma. Inherited PC deficiency and PS deficiency have been associated with venous thrombosis. Both heterozygous PC and PS deficiency appear to be inherited in an autosomal dominant manner in some families. Homozygous PC deficiency presents as neonatal purpura fulminans and results in massive venous thrombosis of the skin and other organs within the first few days of life. Symptomatic heterozygous PC deficiency and PS deficiency have been treated with oral anticoagulants, successfully minimizing recurrence of thrombosis. Coumarin-induced skin necrosis, a rare complication of oral anticoagulant therapy usually seen within three to five days of initiation of therapy, has also been associated with heterozygous PC deficiency. The short half-life of PC (six to eight hours) compared with most of the vitamin K-dependent clotting factors (greater than 30 hours) is the probable reason for this paradoxical response to oral anticoagulants in some PC-deficient patients, since a transient imbalance of procoagulant and anticoagulant factors may exist during initiation of oral anticoagulant therapy. Acquired deficiency of the PC pathway occurs in disseminated intravascular coagulation and possibly other diseases such as those associated with a lupus anticoagulant.