Background: Underuse and an inadequate range for the international normalized ratio (INR) for warfarin use are still problems in the management of the patients with atrial fibrillation (AF) in Japan. Methods and Results: From January to July 2009, a total of 7,937 AF patients [5,468 men (68.6±10.0 years) and 2,469 women (72.2±9.0 years)] were registered from 158 institutions for the J-RHYTHM Registry. Overall, 34.2% of the patients were over the age of 75. The associated cardiovascular diagnoses were hypertension in 59.1%, coronary artery disease in 10.1%, cardiomyopathy in 8.3%, valvular heart disease in 13.7% and artificial cardiac valves in 3.1% of the patients. The type of AF was paroxysmal in 37.1%, persistent in 14.4%, and permanent in 48.5%. Overall, 87.3% of patients were taking warfarin (2.9±1.2mg/day), of whom 66.0% had an INR between 1.6 and 2.6, and 35.4% were in the INR range from 2.0 to 3.0 at the time of registration. Aspirin was prescribed in 22.3% of cases. The CHADS2 score was 0 in 15.7% of patients, 1 in 34.0%, and ≥2 in 50.3%. Conclusions: At present, warfarin is used extensively in patients with AF whose stroke risk is relatively low (ie, in Japan) and half of them had CHADS2 scores of 0 to 1 (UMIN Clinical Trials Registry UMIN000001569). (Circ J 2011; 75: 1328-1333)