Lateral internal sphincterotomy proved to be effective in 99 per cent of the 418 procedures in which it was performed. The complication rate of 5 per cent was an improvement over most of the rates reported in the medical literature and over our previous experience with posterior sphincterotomy and anoplasty. Anal wounds and fissures healed faster than with previous procedures. It is encouraging that this simple and rapid procedure has such great efficacy with a minimum of complications.