Background The E/e' ratio of early diastolic mitral inflow velocity (E) and early diastolic mitral annular velocity (e') has been reported to be useful in estimation of left ventricular filling pressure and predicting of prognosis. On the other hand, E/E��� ratio has not been well investigated in patients with atrial fibrillation (AF). The purpose of this study was to investigate the prognostic significance of E/E��� ratio in patients with AF with preserved LV systolic function. Methods 488 consecutive patients (322 men, age: 66짹11 years) with AF with preserved LV systolic function (Ejection fraction (EF) >50%) were included. The exclusion criteria were defined as the presence of significant other valvular disease, history of cardiac surgery, ischemic heart disease, tachycardia and bradycardia. EF was measured by modified Simpson���s method. E and E��� were measured in 5 consecutive heart beats and averaged. Results Mean follow-up duration after enrollment was 82.5짹4.2months. Cardiac events (Cardiac deaths and re-hospitalization due to heart failure) were occurred in 33 patients. There were no significant differences between the patients without cardiac event group and with cardiac event group in terms of age, gender, heart rate, blood pressure and LVEF. E/E��� ratio in patients with cardiac events was significantly higher than that in patients without cardiac events (14.5짹3.89 vs 10.8짹3.24, p<0.001). Kaplan-Meier analysis showed patients with high E/E��� ratio (>15.0) had poor prognosis (p<0.001 by Log-Rank test). By multivariate logistic regression analysis, E/E��� (x2= 5.47, odds ratio (OR) 1.06, 95% confidence interval (CI) 1.01-1.11, p=0.02) and age (x2= 6.72, odds ratio (OR) 1.05, 95% confidence interval (CI) 1.01-1.11, p=0.03). Conclusion E/E��� ratio is useful prognostic parameter in predicting cardiac events even in patients with AF and preserved LV systolic function.