Purpose: Anticoagulation with vitamin K antagonists (VKAs) provides effective stroke prophylaxis in patients with atrial fibrillation (AF). Current treatment guidelines recommend long-term oral anticoagulation with a VKA, such as warfarin, in patients with AF who are at moderate to high risk of stroke. We conducted a large multicenter survey in Korean patients with AF to gain trend of VKA use. Methods: Eligible patients were adults with AF who had been prescribed VKAs. A total of 4856 patients (mean age 63.6짹12.2 yrs, male 2722 (56.1%)) was reviewed by medical record in 26 hospitals from Jan. 2001 to Oct. 2007. Results: The mean International Normalized Ratio (INR) was 2.04짹0.57, and mean dosage of VKA was 3.7짹1.5 mg. Persons in sixties (1557 patients) were about one third. As patients grew older, INR was more increased and dosage of VKA was more decreased. INR of female patients below sixties was higher than those of male however it was reversed above sixties. The dosage of VKA in male patients was larger than those in female through all age. Patients with INR 2-3 were 44.4% (2146 patients). In addition, patients in sixties were observed within optimal range less than 40 %. Conclusions: In Korean patients with AF, a less than half among them, who were taken with VKA, may reach therapeutic range of INR