Although exercise intolerance is a major symptom of patients with atrial fibrillation (AF), the factors limiting these patients��� exercise capacity remains uncertain. This study evaluated the correlation of the clinical and echocardiographic parameters with the exercise capacity of patients with AF. Seventy-three patients (61 men and 12 women, mean age: 61 years) with chronic AF were included in this study. Those patients with primary valvular diseases were excluded. Standard 2-dimensional and Doppler echocardiography was performed, and we averaged 10 consecutive measurements of each variable. The patients then underwent a symptom-limited treadmill exercise testing. We also measured the patients��� plasma levels of B-type natriuretic peptide before the exercise testing. Of all the clinical and echocardiographic parameters we assessed, age (r=-0.45, p=0.006), the ratio of early mitral inflow velocity to the mitral annular velocity (E/E���) (r=-0.35, p=0.032), and the baseline heart rate were independent predictors of exercise capacity on multivariate regression analysis. In conclusion, the patient���s age, averaged E/E���, and the baseline heart rate provided useful information on exercise intolerance in patients with AF. The E/E���, a non-invasive tool for estimating the LV filling pressure, may especially have important value for predicting functional capacity in this population as it has in individuals with sinus rhythm.