A total of 1355 patients underwent internal sphincterotomy for chronic fissure in ano between 1980 and 1985. Surgical data were obtained for 1102 patients, and 829 patients responded to a questionnaire. Of the 1057 for whom the time of healing was recorded, 1033 (97.7 per cent) healed by a mean time of 5.6 weeks. No significant differences in satisfaction with the outcome or in deficits in continence were noted between groups undergoing lateral, bilateral or posterior midline sphincterotomy. Excision of the fissure was found to be unnecessary. According to responses on the questionnaires, deficits in continence ranging from 'sometimes' to 'frequently' included lack of control of flatus (35.1 per cent), soiling of underclothing (22.0 per cent) and accidental bowel movements (5.3 per cent). A significantly higher proportion of patients who had accidental bowel movements were aged over 40 years.