The continuing controversy between proponents of open and closed commisurotomy might be clarified by analysis of late follow-up with modern actuarial techniques that provide a true perspective of patient risk. Open mitral commissurotomy was used exclusively for 15 yr in 100 patients. There was 1 operative death from pancreatitis and 1 late death from cancer; the actuarially projected survival rate (.+-. the SE) at 10 yr was 97% (.+-. 2). Thirteen patients had preoperative emboli, 6 of whom were in sinus rhythm and 7 in atrial fibrillation. Two patients had postoperative emboli, both in sinus rhythm. The actuarial chance of remaining free of embolism at 10 yr was 97% (.+-. 2). Sixteen patients required reoperation on the mitral valve for functional deterioration. The remaining survivors were in Class 1 or 2 when last seen. The actuarial chance of not requiring a reoperation after 5 yr was 91% (.+-. 4) and at 10 yr, 38% (.+-. 16). Results in different centers are difficult to compare for many reasons, but imprecise statistical methods further obscure such comparisons. The use of actuarial techniques may help to define the role of open mitral commissurotomy.