Twenty patients with chronic anal fissure were randomized into two groups. Ten patients were treated with lateral subcutaneous sphincterotomy and 10 with anal dilatation. Anal dilatation was carried out preoperatively, and at 1 and 3 months after the operation in all patients. Preoperatively there was a significantly increased maximal resting pressure in the 20 fissure patients (80 mmHg median) compared with 20 control subjects (50 mmHg median). Postoperatively a significant decrease in pressure occurred in the dilated group (49 mmHg median p less than 0.05), whereas the pressure was not significantly reduced in the group that underwent sphincterotomy (65 mmHg median p less than 0.05). At 1 year three patients complained of recurrent symptoms of anal fissure in the dilated group compared with one in the sphincterotomy group. Minor continence disturbance was noted in two patients in both groups.