One hundred and three patients with anal fissures have been treated by lateral internal sphincterotomy performed by a subcutaneous myotomy technique, and 99 have been successfully followed up for from 3 to 24 months (mean 11 months). The operation was free from significant complications, apart from slight reactionary bleeding in one case and a perianal abscess in another. The fissure persisted or recurred in three cases, in one of which a further lateral sphincterotomy was performed, with cure. Various minor defects of anal continence were noted in 12 cases. These results have been found to be superior to those obtained previously in the same surgical unit with open posterior internal sphincterotomy and simple sphincter-stretching respectively in the treatment of anal fissure. The most reliable and satisfactory operation at present available for this condition seems, therefore, to be lateral subcutaneous internal sphincterotomy.