The goal of oral anticoagulation therapy with warfarin is to maintain INR values within the therapeutic range in order to prevent complications such as bleeding and thrombosis. The purposes of this study were to investigate the current level of anticoagulation control using INR values, to investigate the incidences of thromboembolism and bleeding complications, and to compare the effect of low intensity INR regimen with therapeutic range recommended by ACCP (American College of Chest Physician). Two hundred three patients with mechanical heart valve replacement done at Yonsei University Cardiovascular Center between January 1994 and December 1996 were selected and reviewed retrospectively. The target INR ranges of $2.5sim3.5$ (ACCP standard) and low intensity INR of $2.0sim3.5$ were used for evaluation. According to ACCP standard, $51.2\%$ of patients and $31.1\%$ of INR values were within the therapeutic range when average INR and cumulative INR were used, respectively. Applying low intensity INR values of $2.0sim3.5$, the therapeutic control was achieved in $57.4\%;and;90.1\%$, using average INR and total INR, respectively. The incidences of major and minor bleedings were $0.5\%;and;26.6\%$, respectively. The incidence of thromboembolism was $0.5\%$. There was no significant difference in terms of complication incidences between INR $2.0sim2.5;and;INR;2.5sim3.5$ groups. However, INR values at the time of bleeding were generally high. In conclusion, the evaluation of patients with mechanical heart valve replacement showed low level of therapeutic control with warfarin therapy. This is partially explained by the fact that the physicians at Yonsei University Cardiovascular Center were using lower intensity INR values as a goal than recommended INR. Also, in the near future, systematic anticoagulation service should be implemented at various hospitals in Korea so that patients on anticoagulant therapy can be more closely monitored to be within the recommended INR by ACCP.