Hereditary deficiencies of coagulation inhibitors like antithrombin III, protein C and protein S lead to an enhanced incidence of thromboembolic complications. Recently, acquired deficiencies of protein S were described in several disease states in which thromboembolic complications frequently occur. These acquired protein S deficiencies reach--in part--the extent realised by hereditary protein S deficiency. Thus, acquired protein S deficiencies seem to be one source of thromboembolic complications occurring in nephrotic syndrome, acute phase reactions, malignancy and pregnancy. In this presentation disease states accompanied by acquired protein S deficiency and the mechanisms leading to these alterations are discussed.