The case of an adult patient with moderately severe protein C deficiency (antigen 16%, activity 12%) is reported. Both parents had protein C levels compatible with heterozygous deficiency. Unlike other reported cases of severe protein C deficiency in adults, the onset of thrombotic symptoms occurred at 1 month of age; however, a symptom-free period until age 17 followed. Replacement therapy with a monoclonal antibody purified protein C concentrate was carried out during the initiation of oral anticoagulation after a course of i.v. heparin for deep vein thrombosis. The administration of the concentrate allowed maintenance of protein C above 50% until a stable therapeutic anticoagulation level could be obtained. This was reached within a short time, thus allowing safe administration of a loading dose of warfarin. We conclude that this approach to the prevention of skin necrosis seems more rapid and safer than previous schedules of oral anticoagulation in protein C-deficient patients.