Background Elevation of hemostatic markers may account for the increased risk of ischemic stroke in patients with non-valvular atrial fibrillation (NVAF). The aim of this study was to determine the effects of anticoagulation intensity on hemostatic markers in patients with NVAF. Methods and Results In 509 patients with NVAF, comprising 263 patients treated with warfarin and 246 patients without warfarin, the hemostatic markers of prothrombin fragment F1.2 (F1.2), fibrin D-dimer, platelet factor 4 (PF4), and β-thromboglobulin were determined and compared with those in 111 patients with sinus rhythm. F1.2 was inversely related with anticoagulation intensity and D-dimer increased with age. All hemostatic markers, except F1.2, were greater in patients with NVAF than in patients with sinus rhythm. F1.2 and D-dimer were significantly lower in patients with international normalized ratio (INR) ≥1.5 than in NVAF patients without warfarin and were not different between NVAF patients with INR of 1.5-1.9 and with INR ≥2.0. Conclusions Low intensity of anticoagulation (INR 1.5-1.9) suppresses the elevated concentration of F1.2 and D-dimer in patients with NVAF, and might be favorable in Japanese patients with NVAF in view of the balance between prevention of thromboembolism and the adverse effect by warfarin (ie, bleeding). (Circ J 2004; 68: 29 - 34)