A multicenter, randomized clinical trial was conducted from March 1996 through January 2001 comparing warfarin therapy for three years to warfarin therapy for three months. The study was performed at thirteen participating hospitals in Oklahoma. The 600 hundred patients were recruited, randomized to either warfarin therapy for three years or warfarin therapy for three months and followed for at least three years. The primary outcome measure was total mortality. Secondary outcome measures included vascular death, objectively documented recurrent venous thromboembolism, and bleeding complications (major and minor). Additional specific aims included: determining the prevalence of antiphospholipid antibodies, antithrombomodulin antibodies, and activated protein C resistance in consecutive patients with proximal-vein thrombosis; determining if these measures, and/or the level of activated factor VII were associated with an increased risk of recurrent venous thromboembolic events, and if so, whether these events were reduced by continuing long term warfarin treatment. These latter specific aims were achieved by performing the laboratory parameters concurrently in patients entered into the clinical trial, and relating these measures to the clinical outcomes on long-term follow-up.