Manometric investigations in patients with primary chronic fissure in ano were performed before and after stretching or sphincterotomy in a randomized clinical trial. Length of the functional anal canal was not influenced by the procedures. At a six-month follow-up, the maximum resting anal pressure was significantly lower in both groups. After stretching and sphincterotomy, the site of maximum pressure in the anal canal had moved orally. This demonstrates that an elevated resting anal pressure is one of the pathogenetic mechanisms for the development of a primary chronic fissure. It is successfully cut off by either stretching or sphincterotomy.