RECURRENT THROMBOSIS VERSUS ANTICOAGULANT-RELATED BLEEDING: The optimal duration of anticoagulation for venous thromboembolism depends on the balance between the risk of thrombosis if anticoagulation is stopped, and the risk of bleeding if patients remain on treatment. In the past decade, five large well designed trials have been completed which have compared different durations of anticoagulation for the treatment of various categories of patients with venous thrombosis. In conjunction with the findings of a number of other prospective studies, these trials have helped to identify risk factors for recurrent venous thrombosis and anticoagulant-related bleeding, and have led to a better understanding of the optimal duration of therapy for individual patients. RISK OF RECURRENT THROMBOSIS: The risk of recurrent thrombosis is low if thrombosis was precipitated by a major reversible risk factor such as surgery. Patients with idiopathic thrombosis (no apparent risk factor) and those with persistent risk factors (e.g., cancer), have a high risk of recurrence. Some hereditary (e.g., protein C, S or antithrombin deficiency) and acquired (e.g., antiphospholipid antibodies) thrombophilic states are risk factors for recurrence independently of whether thrombosis was, or was not, provoked by a major risk factor. DURATION OF THERAPY: Patients with a low risk of recurrence should be anticoagulated for three months. Others should be treated from 6 months to indefinitely, depending on the balance between the risk of recurrence and the risk of bleeding in each individual patient.