Objective: This study was designed to evaluate the influence of chronic atrial fibrillation (AF) on the left atrial (LA) compliance, hemodynamics, and exercise tolerance in patients with severe mitral stenosis (MS). Methods: 1) We retrospectively analyzed the echocardiographic and cardiac catheterization data in 356 patients (74 men and 282 women, mean age 40 years) with MS undergoing percutaneous mitral commissurotomy. 2) We prospectively performed exercise echocardiography in 25 patients (2 men and 23 women, mean age 48 years) with pure MS of more than moderate degree. Mitral valve area, transmitral pressure gradient, pulmonary artery pressure, stroke volume and left atrioventricular compliance were measured. Results: 1) Although normal sinus rhythm group (N=225) and AF group (N=131) had comparable mitral valve area, AF group showed lower transmitral pressure gradient (14.4짹6.7 vs 19.7짹6.9 mmHg, p<0.001), pulmonary artery systolic pressure (45.8짹17.8 vs 52.4짹18.9 mmHg, p=0.002) and cardiac index (2.5짹0.6 vs 3.3짹0.9 L/min/m2, p<0.001). LA volume was higher in AF group (126.0짹61.8 vs 74.1짹26.9 ml, < 0.001) whereas mean LA pressure was higher in sinus rhythm group (20.7짹7.4 vs 25.7짹7.8 mmHg, p<0.001), which suggests the difference of LA compliance between two groups. A linear relationship between LA pressure and LA volume, and between pulmonary artery systolic pressure and LA volume was observed only in patients with sinus rhythm. 2) Baseline mitral valve area, transmitral pressure gradient, stroke volume and pulmonary artery pressure were not different between AF group (N=10) and sinus rhythm group (N=15). However, AF group showed higher LA area (46.3짹11.4 vs 36.2짹6.5 cm2, p=0.01) and atrioventricular compliance (6.1짹1.0 vs 4.7짹1.9 ml/mmHg, p=0.06). Although exercise duration was not different, postexercise pulmonary artery pressure (61짹12 vs 74짹14 mmHg, p=0.03) and stroke volume (51짹11 vs 62짹16 ml, p=0.07) were lower in AF group. Conclusion: Change of LA compliance in chronic AF blunted the increase of pulmonary artery pressure and stroke volume during exercise in the presence of similar degree of MS.