Oral anticoagulant therapy is effective for reducing the risk of thromboembolic events in patients with atrial fibrillation or other heart diseases. However, the intensity of oral anticoagulation therapy required in high risk patients, especially in Japanese patients, to achieve the best balance between the prevention of thromboembolic events and bleeding complications remains unclear. The multicenter study of Toyama Warfarin Rational Dosage (TOWARD) was started in 1996 to determine the optimal level of anticoagulant therapy. This study investigated the relationship between values of thrombotest (TT) and International Normalized Ratio (INR) measured from the same samples to clarify inter-institute variations. The relationship between TT and INR was not linear but hyperbolic. Changes of INR to TT are relatively small in the TT range of more than 20% as compared with the range of 20% or less. There were considerable inter-institute variations of TT, and the coefficient of variation (CV) was 0.16 and 0.24 in the low level and high level anticoagulation samples, respectively. However, the variations became significantly small when the same reference was used. The CV of INR was 0.12 and 0.08 in the high level and low level anticoagulation samples, respectively, and very similar with the control samples without anticoagulation (0.11). The variation was small when INR was obtained from the international sensitivity index (ISI) of thromboplastin less than 1.5. TT is widely used for monitoring oral anticoagulant therapy in Japan, and is an excellent system with little inter-institute variation when a standard reference is offered. Since INR has been established as an international monitoring system, the use of INR measured with thromboplastin of small ISI is recommended for monitoring.