Internal sphincterotomy is thought by most surgeons to have minimal complications. We retrospectively reviewed 306 patients following internal sphincterotomy to determine the incidence of any complications. Major complications (requiring reoperation) caused by fistula, bleeding, abscess, or unhealed wounds occurred in ten patients (3 percent). Minor complications caused by pruritus, persistent wound, pain, bleeding, abscess, discharge, urgency, impaction, or defects of continence occurred in 110 patients (36 percent). Complications were lowest for closed sphincterotomy (20 percent) and highest for open sphincterotomy alone (55 percent). All patients were cured of anal fissure or stenosis. Long-term follow-up (average 4.3 years) revealed a 22 percent incidence of persistent minor complications. Defects in continence caused 15 percent of total long-term morbidity. Minor complications occur frequently after internal sphincterotomy for anal fissure and stenosis. Closed sphincterotomy has the lowest complication rate. Long-term minor defects in continence occur in a significant number of patients.