Background: Medication adherence (MA) is the key for the treatment of many cardiovascular diseases. MA for warfarin is more important than for other drugs due to its narrow therapeutic range and fatal adverse reactions. This study aims to investigate the actual rate of MA in the patients taking warfarin, its influencing factors including knowledge and self-efficacy and its relationship with INR. Methods and Results: Patients taking warfarin who had visited the outpatient clinic of the Cardiovascular Center, affiliated with Y University located in Seoul, from April 9 to 30, 2007, were selected as research subjects, and a selective sampling of 230 patients who could respond to the survey was carried out. Of those, a total of 204 responses were ultimately used. For examining the actual conditions of MA and its factors, a survey was used, and INR results and factors influencing MA were partially examined through reviewing medical records. The data were statistically analyzed using SAS 8.2 Version, and the results are as follows. When MA was defined as taking warfarin following medical advice (frequency, dosage, time, and precautions) the ratio of MA was 56 people among 204 (27.45%) and non-medication adherence (non-MA) was 148 (72.55%). Adherent group had greater understanding about wafarin than non-adherent group measured by 10 items (7.20짹1.69 vs 6.55짹1.83, p���0.05) and had more significant self-efficacy (p���0.000). The patients who got an education session offered by the nurse in a group had also a higher knowledge score than who had not (p���0.000). However, MA did not affect the ratio of the patients who have appropriate INR. Only among the patients who got a valve surgery, non-adherent group had higher ratio of increased INR beyond the proper target (p���0.05). Conclusions: The results show that knowledge of medication and self-efficacy exerts significant influence on MA. Based on this, we suggest the development of an educational program to increase knowledge of medication and self-efficacy, the factors to increment low MA in patients taking warfarin.