Keyword : oral anticoagulation, time in therapeutic range, atrial fibrillation Backgroud : Oral anticoagulation (OAC) therapy is effective in atrial fibrilallation (AF) and needs careful and strict monitoring of the therapeutic range of the international normalized ratio (INR). The time in therapeutic range (TTR) is a quality measure for vitamin K antagonists management and presents the proportion of time within INR. Several studies have reported TTR achieved in various clinical settings. A meta-regression analysis of recently published studies reported a mean TTR in all studies of 64%. However, wide variation in TTR was reported in a recent clinical trial of OAC in AF. The object of this study is to investigate indiviaudl TTR of warfarin management in patients with atrial fibrillation according to the international standard for the management of OAC. Method and materials : Total 98 patients of cardiology and neurology department were included. Inclusion criteria for analysis is as follows : 1) diagnosis : nonvalvular atrial fibrillation, 2) patients prescribed only on outpatient department, 3) follow-up duration : less than 56 days, 4) duration of warfarin prescription : at least 90 days. Exclusion criteria follows as : 1) mechanical valve replacement, 2) follow-up duration : more than 56 days, 3) hospitalization or operation, 4) venous thrombosis. Target INR was 2.0 to 3.0. Rosendaal method as TTR calculation method was adopted, which uses linear interpolation of INR values in each patient receiving OAC to calculate the percentage of days when the INR is in the therapeutic rnage (2.0-3.0). Other TTR measures except the patient-time approach were excluded. Results : Baseline characteristics are as follows : mean age 69 years, men 60 %, female 40 %. Mean TTR of total 98 patients was 41.1 % and mean follow-up duration 37 days. Mean TTR of 43 patients in cardiology department was 38.58 % and mean follow-up duration 34 days. Mean TTR of 55 patients in neurology department was 43.5 % and mean follow-up duration 40 days. Conclusion : Mean TTR of patients with atrial fibrillation was 41.1 % and quite lower than the value recently published in a meta-analysis.