PURPOSE: The number of patients who take warfarin is growing and so is the number of complications. Hemorrhage is the major complication, but the clinical characteristics and outcomes have not been determined for Korean patients. Therefore, we tried to evaluate the characteristics of the patients with hemorrhagic complications after taking warfarin as anticoagulation therapy. METHODS: We retrospectively reviewed the medical records of the patients who visited the emergency room with bleeding complications after taking warfarin anticoagulation at the out-patient clinic for 1 year from 1st January 2008. We compared between two groups (the major hemorrhage group vs. the minor hemorrhage group) according to the clinical criteria, the unstable vital signs that required blood transfusion, transfusion more than 2 units of blood, the need for further laboratory follow-up, the need for interventional treatment and the development of critical complications or death due to bleeding. RESULTS: There were 150 patients who met the criteria and had acute hemorrhagic complications (the major group: 90 patients and the minor group: 60 patients). In the major hemorrhage group, the frequent sites of bleeding were the gastro-intestinal system (40 patients), lung (14 patients) and intracranium (7 patients). At the emergency room, the major group showed a higher initial INR of the activated prothrombin time than did the minor group (p=0.02). The bleeding sites of the fatal cases were the gastro-intestinal system (3 patients), lung (3 patients) and intracranium (3 patients), but the percentage of fatality was the highest for intracranium bleeding. CONCLUSION: In the major hemorrhage group, gastrointestinal bleeding was the most frequent complication and fatality was the highest for intracranium bleeding. An initially higher INR showed a greater risk of major bleeding, but not more fatalities.