Although the exact role of PS in vivo in the protein C anticoagulant pathway has not been clarified, there is no doubt that PS is an important anticoagulant protein and that PS deficiency is primarily associated with venous thromboembolism. The reported prevalence of PS deficiency in thrombosis patients varies between 1.5-7%, the difference being due to methods used and to the selection of patients. Clinical symptoms in patients affected with PS deficiency are very similar to those with protein C deficiency (DVT). Superficial thrombosis seems to be more common than for antithrombin deficient patients.