Objectives: Warfarin is a representative anticoagulant, and it has been the preferred drug for treating thromboembolic diseases and preventing ischemic stroke. It should be administered in a delicate manner, because combined therapy with other drugs could affect the INR (International Normalized Ratio), thus bleeding tendency might occur. Interaction with herbal medicines still remains controversial. We aimed to examine the effect of herbal medicine on INR in patients being treated with warfarin. We aimed to examine the effect of herbal medicine on INR in the patients being treated with warfarin. Methods: We enrolled subjects being treated with a combined therapy of warfarin and herbal medicine from 1 October, 2002 to 20 November, 2003 at the Department of Cardiovascular & Neurologic Diseases (Stroke Center), Hospital of Oriental Medicine, Kyung Hee Medical Center. INR was monitored per 1 to 2 weeks and the significance of change was examined by Wilcoxon signed rank test or repeated measure analysis Results: Eighty-three subjects were included in the final analysis. As a whole, their INR was not significantly changed. There were only 9 cases in which INR changed more than 1.0; 7 showed increasing tendency, and 2 showed decreasing tendency of INR. Among the 9 cases, only 5 were suspected to have a relation to herbal medicine. Conclusions: We suggest that there was no strong evidence that herbal medicine has an influence on INR, but there is a need to perform further evaluation on larger samples to reach a concrete conclusion.