Background: Determinants of warfarin use and anticoagulation levels in atrial fibrillation (AF) patients have not been clarified thoroughly. Methods and Results: A total of 6,324 patients with non-valvular AF and congestive heart failure, hypertension, age, diabetes, prior stroke (CHADS₂) score ≥1 were used to investigate determinants of warfarin use, and 6,932 patients with AF receiving warfarin were used to investigate determinants of international normalized ratio (INR) of prothrombin time. Target INR levels for non-valvular AF patients were 1.6-2.6 for patients aged ≥70 years and 2-3 for patients aged <70 years. Those for patients with valvular AF were 2-3. Patients with non-valvular AF and CHADS₂ score ≥1 receiving warfarin (n=5,614) more frequently had permanent AF, congestive heart failure, and stroke or transient ischemic attack (TIA), and had higher CHADS₂ scores than those not receiving warfarin. Determinants of warfarin use were age (≥60 years), AF type (persistent and permanent), and comorbidities (congestive heart failure, diabetes mellitus, and prior stroke or TIA). Use of antiplatelet drugs was a negative determinant of warfarin use. Only 53% of patients met the target INR levels. Determinants for the meeting of the target INR level (vs. lower INR level) were age (≥60 years), permanent AF, hypertension, and prior stroke or TIA. Use of antiplatelet drugs was a negative determinant of the INR level. Conclusions: Currently in Japan, adherence to the guidelines regarding anticoagulation therapy is limited (UMIN Clinical Trials Registry UMIN000001569). (Circ J 2011; 75: 2357-2362)