OBEJCTIVE: Warfarin is an oral anticoagulant drug that needs continuous clinical and laboratory monitoring due to its narrow therapeutic index and life-threatening complications. This study aims to assess knowledge and adherence of patients to their warfarin treatment regimen and to identify barriers that prevent patients to take their warfarin therapy regularly. METHODS: In this cross-sectional study, a systematic random sample of 307 patients was selected from Alshaab teaching hospital heart section and Ahmed Gasim Hospital. Data was collected by anonymous interview-based questionnaires. RESULTS: Male patients (52.5%) exceeded females (44.8%), the mean age was 48.79. About 57.98% of the studied patients had moderate overall knowledge score, and more than half of them had good adherence levels (62.2%). The study shows that: Forgetting (43.7%) was the main barrier preventing the patients from taking their medication, followed by drug unavailability (36.8%) and high cost (19.5%). There are statistically significant differences between patient’s age/education and their level of knowledge (p=0.008/p=0.011). The correlations between patients’ adherence to warfarin oral anticoagulant and their level of knowledge is statistically insignificant (r=0.647, p=0.739). There are significant association between patients' level of adherence and the time from which the patient starts to take warfarin (p-value= .034). CONCLUSION: The majority of the studied patients had moderate overall knowledge score about Warfarin oral anticoagulant, and more than half of them had good adherence levels. “Forgetting” was the main barrier preventing the patients from taking their medication. DESCRIPTORS: Warfarin, Knowledge, Adherence, Barriers.