A large left atrial (LA) volume, which represents chronic diastolic dysfunction, is associated with a poor outcome, regardless of systolic function. Thus, the LA volume provides a long-term view of whether the patient has diastolic dysfunction, regardless of the loading conditions present at the examination, such as hemoglobin A1c in diabetes mellitus. To date, the relation between the LA volume and left ventricular (LV) filling pressure has not been confirmed directly by simultaneous echocardiographic catheterization. The present study, therefore, assessed the correlation between the LA volume and LV filling pressure in patients with severe mitral regurgitation (MR). Because the LA pressure increases to maintain adequate LV diastolic filling, increased atrial wall tension tends to dilate the chamber and stretch the atrial myocardium. Therefore, the lower the ability of the left atrium to stretch, the greater the pressure in the left atrium. The study is designed to assess 1) the relationship between LV filling pressure and LA distensibility, and 2) the power of left atrial distensibility to predict the prognosis, including operation mortality, the rate of post-operation atrial fibrillation, and late heart failure event in patients with severe mitral regurgitation.