Randomization is stratified for secondary versus idiopathic thrombosis and for center. After completing the first uneventful three months of anticoagulation, patients will be randomized to fixed durations of warfarin or to flexible durations. In the fixed duration group, patients with idiopathic deep-vein thrombosis will receive 3 additional months of treatment (for a total treatment duration of 6 months), and patients with secondary deep-vein thrombosis will discontinue treatment (for a total treatment duration of 3 months). In the flexible duration group, patients will have ultrasound at the time of randomization and then at 3, 9, 15 and 21 months if the index event is idiopathic or at 3 and 9 months if the index event is secondary. Anticoagulant therapy will be discontinued if the thrombosis has recanalized. If residual venous thrombosis is detected, treatment will be continued until the following ultrasound testing, and for a maximum of 9 months for secondary and 21 months for idiopathic venous thrombosis. In both groups, warfarin will be adjusted to a target international normalized ratio of 2.0 to 3.0.