Protein S, which circulates in plasma in a free and bound form, is an anticoagulant protein that stimulates both activated protein C (APC) and tissue factor pathway inhibitor (TFPI). Hereditary type I protein S deficiency (low total and low free protein S) is a well-established risk factor for venous thrombosis, whereas the thrombosis risk associated with type III deficiency (normal total and low free protein S) has been questioned.Kaplan-Meier analysis was performed on 242 individuals from 30 families with protein S deficiency. Subjects were classified as normal, type I deficient or type III deficient according to their total and free protein S levels. Genetic and functional studies were performed in 23 families (132 individuals).Thrombosis-free survival was not different between type I and type III protein S-deficient individuals. Type III deficient individuals were older and had higher protein S, TFPI and prothrombin levels than type I deficient individuals. Thrombin generation assays sensitive to the APC- and TFPI-cofactor activities of protein S revealed similar hypercoagulable states in type I and type III protein S-deficient plasma. Twelve PROS1 mutations and 2 large deletions were identified in the genetically characterized families.Not only type I, but also type III protein S deficiency is associated with a hypercoagulable state and increased thrombosis risk. However, these findings may be restricted to type III deficient individuals from families with mixed type I/III protein S deficiency, as these represented 80% of type III deficient individuals in our cohort.