Prospective controlled randomized clinical trial. Consecutive patients with acute proximal deep vein thrombosis (DVT) of the lower extremities, with or without contemporary manifestations of pulmonary embolism, are randomized to receive either a fixed duration of anticoagulant therapy (three months in patients with DVT secondary to transitory risk factors, six months in patients with idiopathic DVT) or a flexible duration of anticoagulant therapy, according to the persistence of residual thrombi, as shown by leg veins ultrasonography (up to 1 year in patients with secondary DVT, up to 2 years in those with idiopathic DVT). All patients are followed up to three years to assess the development of (objectively documented) recurrent thromboembolism. The rate of recurrent thromboembolism is compared between the two study groups, as well as the rate of major bleeding complications occurring during anticoagulation.