BACKGROUND: Atrial fibrillation (AF) is one of the most common clinical arrhythmia. AF may cause disabling symptoms and serious adverse effects, such as impairment of cardiac function or thromboembolic events. Until now, there were no study about the clinical characteristics of atrial fibrillation throughout this nations. The purpose of this study is a establishment of epidemiologic database of patients with atrial fibrillation in this nations. METHODS: 867 patients from 12 university hospitals were involved in this study. Atrial fibrillation were diagnosed with documented ECG in all patients. Medical history, physical findings, basic Laboratory finding, ECG, echocardiography and 24-hour Holter monitoring of these patients were evaluated. Chronic atrial fibrillation were defined as the duration of AF longer than 72 hours regardless of intervention. RESULTS: In patients with chronic atrial fibrillation, the most common symptom was dyspnea and the most common associated diseases were valvular heart disease, hypertension and ischemic heart disease. 2) In patients with paroxysmal atrial fibrillation, the most common symptoms were palpitation, dizziness and chest pain, and the most common associated diseases were hypertension, ischemic heart disease and valvular heart disease. 3) left atrial size, systolic and diastolic left ventricular size in patients with chronic atrial fibrillation were significantly increased as compared those in patients with paroxysmal atrial fibrillation (p<0.001). 4) cardiomegaly and pulmonary edema were more common in patients with chronic atrial fibrillation (p<0.0001). CONCLUSIONS: This study is first large multicenter study about atrial fibrillation in this nations. These data can be used as basic data for follow up and management of atrial fibrillation.