A prospective study of 18 consecutive patients undergoing unilateral, partial-thickness, distal, internal sphincterotomy for the treatment of chronic anal fissure was performed. Biopsies were taken from the base of the fissure and from the lateral muscle before division. Normal specimens were taken from the internal anal sphincter of patients undergoing abdominoperineal resection. Specimens confirmed the presence of fibrosis throughout the internal anal sphincter in patients with anal fissures, but none in controls.