One hundred and thirty cases of closed mitral commissurotomy were followed for up to 25 years and 10 months. There was no operative death, but 31 cases died during the follow-up period. Eight cases died suddenly of unknown cause, 7 due to heart failure, 5 due to thromboembolism, 4 on reoperation, and 6 due to other reasons. In the 7 cases who died of heart failure late after commissurotomy, 3 cases refused reoperation. Each of the remaising 4 cases were not operated on because of associated severe liver dysfunction, left ventricular dysfunction plus pulmonary hypertension, respiratory failure due to bronchial asthma, and unknown reasons, respectively. The actuarial survival rate was 93.6% 10 years after surgery, and 72.2% 20 years after surgery. Forty-two cases had reoperation with a mean interval of 12 years and 6 months. Reoperation-free survival rate was 88.7% 10 years after the first operation and 42.8% 20 years after the first operation. Incidence of major thromboembolism was 1.25%/patient-year. Thromboembolism and sudden death of unknown cause constituted the leading cause of late death and played a key role in long term results. Cardiac event-free survival rate was 65.7% 10 years after surgery and 32.6% 20 years after surgery. From these results it was concluded that the clinical limitations of the effectiveness of closed mitral commissurotomy was around ten years after surgery. We believe that these findings provide useful information for percutaneous transvenous mitral ommissurotomy.