Background:Warfarin is widely used for prevention of thromboembolism in patients with valvular atrial fibrillation (AF), and an international normalized ratio (INR) of prothrombin time between 2.0 and 3.0 is recommended. Optimal intensity of anticoagulation with warfarin in Japanese patients with valvular AF, however, has not been clarified thoroughly as yet.Methods and Results:We evaluated the status of anti-thrombotic therapy and incidence rates of events in 410 patients with mitral stenosis and/or mechanical valve replacement (valvular AF) among 7,816 patients with AF followed in the J-RHYTHM Registry. Patients were divided into 5 groups based on INR (<1.6, 1.6–1.99, 2.0–2.59, 2.6–2.99, and ≥3.0) at the time of event or at the end of follow-up in order to determine the target INR for patients with valvular AF. Warfarin was prescribed in 407 (99.3%) of valvular AF patients. During a 2-year follow-up period, thromboembolism and major hemorrhage occurred in 12 (2.9%) and in 15 (3.7%) patients, respectively. Among patients receiving warfarin, 2-year incidence rates of thromboembolism were 10.3%, 1.6%, 0.6%, 3.0%, and 0.0% (P=0.003 for trend), and those of major hemorrhage were 1.5%, 1.6%, 3.2%, 6.1%, and 21.1% (P<0.001 for trend), respectively.Conclusions:INR between 1.6 and 2.6 could be optimal to prevent thromboembolism without increasing major hemorrhage in Japanese patients with valvular AF. INR 2.6–2.99 would also be effective, but is associated with a modestly increased risk of major hemorrhage. (Circ J 2015; 79: 325–330)