For a 7-year period, cardiac rhythm before and after surgery was determined in 106 patients with mitral stenosis presenting with atrial fibrillation (AF) who had undergone open mitral commissurotomy. Forty-three of the patients reverted to sinus rhythm (SR) after primary or secondary direct-current (DC) cardioversion after surgery and maintained it until discharge from hospital. Thirty patients maintained SR for 3 months to 7.2 years (mean 2.5 years) after surgery. The actuarial maintenance rate of SR was 50% 7 years after surgery in these 43 patients. The duration of AF, preoperative left atrial dimension by M-mode echocardiogram and pathologic classification of the mitral valve were factors supposedly influencing the maintenance of SR for a long period after DC cardioversion. In 30 patients who reverted back to SR and maintained SR late postoperatively, the preoperative duration of AF was up to 5 years, and 35% of the patients had had AF for more than 1 year. Also, in 40% of these 30 patients, the preoperative cardiothoracic ratio was more than 60%. It is concluded that if sinus rhythm is restored by DC cardioversion before discharge from hospital after open mitral commissurotomy, it has a 50% chance of being maintained for 7 years after surgery. Long duration of AF and large cardiothoracic ratio should probably not dissuade one from attempting secondary DC cardioversion in these patients.