The aim of the present study was to evaluate the effect of lateral internal sphincterotomy on pain after open hemorrhoidectomy. From 1998 to 2003, seventy-eight (78) patients with fourth-degree hemorrhoids were included in this prospective randomized trial. The patients were randomized into two equal groups of 39 patients. Patients from group I underwent Milligan-Morgan hemorrhoidectomy. Patients from group II, quite apart from Milligan-Morgan hemorrhoidectomy, underwent lateral internal sphincterotomy up to the dentate line, in the left hemorrhoidectomy wound. One surgeon from the Department, who did not know to which group the patients belonged, evaluated the postoperative course in all the patients. After the first bowel movement, there were three (7.7%) patients who did not experience any pain in the internal sphincterotomy group, while in the non-internal sphincterotomy group all patients experienced mild or moderate pain. There were also more patients who experienced excruciating pain in the non-internal sphincterotomy group than in the internal sphincterotomy group (25 vs. 18); these differences were statistically significant (p = 0.034). There was no significant difference in the Wexner Incontinence Scale between the groups (p = 0.228). The addition of lateral internal sphincterotomy to open hemorrhoidectomy seems to have a positive effect on reducing postoperative pain in a few patients, without affecting the postoperative complications rate.