Anticoagulation is the corner stone of therapy for venous thromboembolism. The optimal duration of this therapy depends on the balance between the risk of recurrent thrombosis if anticoagulants are stopped, and the risk of bleeding if patients remain on treatment. The risk of recurrence 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 (eg, cancer), have a high risk of recurrence. Some hereditary (eg, protein C, protein S or antithrombin deficiency; homozygous factor V Leiden) and acquired (eg, antiphospholipid antibodies) thrombophilic states are also risk factors for recurrence. Three months of anticoagulation is recommended when the risk of recurrence is low, whereas the duration of therapy should be extended to 6 months or longer when this risk is high, depending on the balance between the risk of recurrence and the risk of bleeding in each individual patient. Heparin preparations, at doses intermediate to those used for the acute treatment of venous thromboembolism and for primary prophylaxis, are an alternative to oral anticoagulants during the maintenance phase of treatment.