A prospective randomized trial has compared manual dilatation of the anus (MDA) during general anesthesia with lateral subcutaneous sphincterotomy (LSS) during local anesthesia for the management of anal fissure in 156 patients. The two groups were similar with respect to age, sex and symptoms. There was no difference in the duration of time off work or early complications of treatment but, four months after operation, 93 per cent claimed to have been improved by MDA compared with 78 per cent after LSS (P<0.05). Recurrent fissure was recorded in 13 patients after LSS (29 per cent) compared with four (10 per cent) after MDA (P<0.02). There was a significant reduction in anal pressure at four months (P<0.02) after MDA, (123±31 to 97±33) and LSS (127±36 to 104±32), but the anal pressure remained unchanged by operation in all patients where pressures were measured with recurrent fissure. These data indicate that MDA gives better results than LSS for treatment of anal fissure and that successful treatment is associated with a reduction in anal pressure.