The optimum duration of anticoagulation therapy for deep-vein thrombosis (DVT) and pulmonary embolism (PE) is not clear. We have carried out a multicentre comparison of 4 weeks' and 3 months' anticoagulation in patients admitted to hospital with acute DVT, PE, or both. Of 712 patients enrolled, 358 were assigned 4 weeks' treatment and 354 3 months'. Objective confirmation of the diagnosis was obtained in 71%. PE caused or contributed to death in 7 patients (3 treated for 4 weeks, 4 for 3 months). Adverse effects were uncommon, although 1 patient (4-week group) died of haemorrhage. The numbers of patients whose thromboembolism failed to resolve on treatment was lower in the 3-month group than in the 4-week group (13 [3.7%] vs 24 [6.7%], p = 0.10) as was the number who had recurrences (14 [4.0%] vs 28 [7.8%], p = 0.04). Among patients with postoperative DVT or PE the rate of treatment failure and recurrence was low (2.6%) and there was little difference between the treatment groups. By contrast, among medical patients the rate was 12.8%, with a clear difference in favour of 3 months' treatment. If venous thromboembolism arises after surgery, 4 weeks of anticoagulation should be adequate. In other settings, patients with new DVT, PE, or both, who do not have a persisting underlying cause or risk factor should receive anticoagulants for 3 months.