Background: Although thromboembolism is common and one of the major causes of mortality in cancer patients, maintaining therapeutic anticoagulation effect with warfarin is challenging. This study aimed to determine the prevalence and the causes of non-therapeutic INR (International Normalized Ratio) in cancer patients. Methods: Medical and pharmacy records for cancer patients managed by the pharmacist-run anticoagulation service (ACS) between May, 2010 and April, 2011 at Seoul National University Hospital were retrospectively reviewed. The causes of non-therapeutic INR were identified and compared with the results from a former study with mechanical heart valve patients. Results: A total of 335 cancer patients and 6,737 patient-visits were analyzed producing 68% (n=4,590) of non-therapeutic INR readings. Eighty-five percent of the non-therapeutic INR readings were categorized as sub-therapeutic. Frequent causes linked to non-therapeutic INR included inadequate dosage adjustment (21.8%), changes in health status (11.8%), dietary changes (8.1%), and drug interactions (4.2%). More than half of the non-therapeutic INR values had no known etiology. As causes for non-therapeutic INR, changes in health status (p<0.0001), adverse reactions (p<0.0001), and dietary changes (p=0.017) were statistically more frequent in cancer patients than in patients with mechanical heart valves. Furthermore, exposure to sub-therapeutic INR were more prevalent in cancer patients than in patients with mechanical heart valve (p<0.0001). Conclusions: This study shows that there is a tendency to keep the level of INR low and that health status change, dietary change, and drug interactions are found to be frequent causes for non-therapeutic INR in cancer patients.