A twelve-year follow-up study was carried out with 98 consecutive patients operated on for pure mitral stenosis with open mitral commissurotomy (OMC) or valve replacement with porcine valves (MVR). Each patient had the extent of valve distortion diagnosed preoperatively by two-dimensional echocardiography as either grade I of the dome-formation type, grade II of the intermediate type, or grade III of the mass-formation type. Thirty grade I patients and 24 grade II patients underwent OMC and 44 grade III patients received MVR. The long-term survival and reoperation rates were significantly poorer with grade II than grade I patients and there was no statistical significance between grades II and III. However, the clinical conditions set by NYHA are significantly worse in grade II than in grade III. This can be clearly explained by the fact that the left atrial volume decreasing ratio (LAVDR) in the latest period, which was measured in patients with more than 7 years survival, remained nearly at the early postoperative value in grade III, while the LAVDR of grade II at the latest period grew worse and became insignificantly different from the preoperative values. This study clearly shows that when OMC is applied to grade II patients, frequent reoperations might be necessary because of poor hemodynamic conditions. Thus, it might be better to adopt MVR rather than OMC with grade II patients with moderately distorted mitral valves.