Abstract We report the case of an elderly woman who developed acute colon dilatation as a consequence of a painful fissure of the anus with severe hypertonia. After having excluded mechanical aetiology, a lateral internal sphincterotomy was performed under local anaesthesia. The procedure cured the anal pain and, by facilitating normal bowel function, allowed resolution of abdominal symptoms. The aetiology of acute colonic dilatation in this patient may be related to the painful anal fissure and hypertonia. In the elderly, the presence of simple anal diseases may predispose to the occurrence of life-threatening complications, such as acute colonic dilatation.