Prothrombin fragment 1 + 2 (F1 + 2) and thrombin-antithrombin-III-complex (TAT) levels were compared in 31 orally anticoagulated patients with inferior vena caval filters and a control group of 31 orally anticoagulated patients without caval filters and the incidence of markers of thrombophilia (deficiency of antithrombin-III, protein C, protein S and factor XII, presence of lupus anticoagulants) was determined. 8 of 31 patients (26%) from the group of caval filter carriers showed markers of thrombophilia (3 protein S deficiencies, 1 protein C deficiency, 2 factor XII deficiencies and 2 patients with lupus anticoagulants). In all orally anticoagulated patients a significant interdependence (p < 0.05) between F1 + 2- and TAT-levels and intensity (INR) of the oral anticoagulation could be observed. Comparison of F1 + 2- and TAT-levels of caval filter carriers and controls revealed no significant difference which leads to the conclusion that inferior vena caval filters do not induce detectable systemic activation of prothrombin under adequate oral anticoagulation therapy.