METHODS Study patients. Patients with a first episode of documented unprovoked and provoked proximal DVT will be eligible for the study if they had completed at least three months of OAT (target INR 2.5, range 2.0-3.0). Provoked DVTs are defined as thrombotic episodes associated with pregnancy or puerperium, recent (i.e. within three months) fracture or plaster casting of a leg, immobilization with confinement to bed for three consecutive days and surgery with general anesthesia lasting longer than 30 minutes. Unprovoked DVTs are defined as thrombotic episodes occurred in apparently healthy individuals. Patients with active cancer, limited life expectancy, antiphospholipid antibody syndrome, or an other known thrombophilic status (such as antithrombin deficiency), serious liver disease, renal insufficiency (serum creatinine > 2 mg/dL), those who lived too far from the center will be excluded from the study. The study has been approved by the institutional review boards of all participating centers. All enrolled patients will provide written informed consent.