Background and Objectives: Atrial fibrillation (AF) is a frequent comorbid condition and aggravating factor of acute heart failure (AHF). However, the prevalence, predictors, and impacts of AF on AHF have been poorly studied in Korean patients. Methods: Among 3,200 patients from the Korean Acute Heart Failure Registry, 2,837 patients who had analyzable electrocardiography without ventricular tachycardia or fibrillation at admission were divided into 2 groups; patients with AF (AF group, n=728, 69.7짹13.1 years) versus patients without AF (No AF group, n= 2109, 67.5짹14.5 years). Baseline characteristics and initial clinical outcomes were compared. Results: The prevalence of AF was 30.5% and significantly higher in patients with older age, previous history of HF, valvular heart disease or valve surgery, and cerebrovascular accidents. Among etiologies of AHF, valvualr heart disease and thyroid disease related HF was significantly higher in AF group, but ischemic heart diseases including acute myocardial infarction was significantly lower in AF group. The incidence of in-hospital death or stroke, duration of hospitalization, rehospitalization, and death at 1 month was not different between the groups. Conclusion: The present study demonstrated that AF was common in Korean patients presented with AHF, but AF was not associated with early clinical outcomes.