BACKGROUND: The prognosis of asymptomatic deep vein thrombosis (DVT) is uncertain and there is no consensus on the necessity of detection and treatment.Methods and Results:We retrospectively evaluated 300 patients with asymptomatic lower extremity DVT screened from 4,514 consecutive patients on ultrasound at Kyoto University Hospital between January 2010 and September 2015. The subjects had concomitant active cancer in 40%, unprovoked DVT in 59%, and distal DVT in 70%. The cumulative 5-year incidences of symptomatic recurrent venous thromboembolism (VTE); major bleeding; and all-cause death were 14.5%, 16.6%, and 34.1%, respectively. Among 232 patients (77%) with prolonged anticoagulant therapy, anticoagulants were discontinued in 48.4% at 1 year. Anticoagulant therapy was associated with a significantly higher incidence of major bleeding compared with the non-anticoagulant group (20.5% vs. 1.5%, P=0.01) with no significant effect on the incidence of VTE. In patients with active cancer, the favorable effect of anticoagulants relative to no anticoagulants for VTE was significant (HR, 0.22; 95% CI: 0.05-0.95). CONCLUSIONS: Prolonged anticoagulants therapy was implemented in the majority of patients with asymptomatic DVT, but was associated with a significantly higher risk for major bleeding. On subgroup analysis in patients with active cancer, however, there appeared to be a benefit of prolonged anticoagulant therapy in decreasing the rate of symptomatic recurrent VTE.