Objective: The aim of this study was to assess the frequency and the predictive factors for atrial fibrillation (AF) in patients with severe mitral regurgitation (MR). Materials and Methods: This study is a retrospective evaluation involving of 199 patients (100 females, 99 males, mean age 53.8±18.5) with severe MR who had been admitted to our clinic between 1997 and 2001. Etiologies of MR were; rheumatic heart disease (n=132), ischemic heart disease (n=44), mitral valve prolapsus (n=14), mitral annular calcification (n=9). Patients who had AF (n=95), sinus rhythm (n=98) or recurrent AF (n=6) were determined according to ECG. Age, gender, smoking, hypertension, diabetes mellitus, electrocardiographic left ventricular hypertrophy (LVH), echocardiographic left atrial (LA) diameter, left ventricular end-systolic dimension (LVESD), left ventricular end-diastolic dimension (LVEDD), ejection fraction (EF), right atrial size, pulmonary artery pressure and presence or absence of mitral stenosis were recorded. Results: Atrial fibrillation was observed in 50.8% of the patients. Female gender (60% vs. 40%, p=0.009), LA diameter (5.6+1.0 cm vs. 4.8+0.6 cm, p<0.001) and right atrium size (30.7% vs. 11.2%, p=0.001) were found as the predictive factors of AF in univariate analysis. Multiple logistic regression analysis showed that advanced age (p<0.014), female gender (p=0.02), LA size (p<0.001) and coexistence of MR with mitral stenosis (p<0.013) were independent risk factors for AF in patients with rheumatic MR. None of variables could be predictive for AF in patients with ischemic MR. Atrial fibrillation was found more frequently in patients with rheumatic MR than that of ischemic MR (p<0.001). In rheumatic MR, left atrium cut-off value for AF was found to be 5.5 cm (sensitivity: 52.1%, specificity: 90.7%). Conclusion: Atrial fibrillation is a common arrhythmia in severe MR (50.8 %). Left atrial size is a powerful independent predictor for AF.