It would be important to estimate in advance the risk of recurrent thrombosis. Deficiencies of antithrombin, protein C or protein S, or resistance to activated protein C are associated with a biochemically detectable prethrombotic state. It is thus far unknown whether in patients with a history of thromboembolism but without a defined clotting abnormality a heightened coagulation activation is detectable. We investigated the value of prothrombin fragment F1+2 (F1+2) as a predictor of recurrent venous thromboembolism. Furthermore, we compared the F1+2 levels of thrombosis patients without a defined clotting defect to those of Factor V Leiden patients with a history of venous thrombosis and to those of healthy controls. 180 patients without a defined clotting abnormality and 73 patients with Factor V Leiden were prospectively followed after discontinuation of oral anticoagulants for venous thrombosis and F1+2 was measured at regular intervals. Recurrent venous thromboembolism occurred in 23 (9%) of the 253 patients. Before or at several time points after oral anticoagulants, no significant difference in F1+2 levels was found in patients with and without recurrent thrombosis. F1+2 levels at 3 weeks and prior to recurrence were not significantly different in both patient groups. Over a one-year observation period, F1+2 levels of both patients with and without Factor V Leiden were higher than those of the controls. No difference in F1+2 was seen between patients with and without Factor V Leiden. We conclude that monitoring of F1+2 is not suitable for identification of individuals at risk of recurrent venous thrombosis. Permanent hemostatic system activation is detectable both in patients with a defined abnormality of the clotting system and in patients in whom a particular defect has not (yet) been identified.