Introduction: Anticoagulation therapy with warfarin is monitored by the prothrombin time (PT) assay. The PT is standardized using international normalized ratios (INRs). By keeping the INR within specific values, it is possible to reduce potential complications from the treatment. To increase the convenience for the PT monitoring, point-of-care devices suitable for capillary whole blood measurements have been developed. The purposes of this study were to compare the INR values obtained by such a device, Protime, with those obtained from the Turibimetry-type PT assay. Methods and Results: In 68 consecutive warfarin-treated patients (M;F=58:10, 57.9 짹 12.6 years old) due to persistent atrial fibrillation, INR was measured in capillary whole blood samples with ProTime and was compared to venous plasma samples analyzed with the Turibimetry-PT method. INR values (1.91 짹 0.78) and PT (24.9 짹 10.2 seconds) obtained from ProTime were similar to those (2.23 짹1.01, 20.6 짹 6.6 seconds) from Turibimetry-PT assay (NS). These two values from ProTime produced a correlation coefficient of 0.919 and 0.919 to those from the Turibimetry-PT assay (p<0.001), respectively. Conclusions: INR analysis of capillary whole blood with ProTime is comparable with INR measured in plasma with Turibimetry. ProTime might facilitate long-term PT monitoring.