Perforation of the rectum requires early recognition and treatment. The diagnosis of rectal perforation is sometimes difficult owing to non specific clinical presentation, especially in elderly patients, in whom, in case of acute abdomen, Computed Tomography (CT) is increasing used as first diagnostic procedure [1]. Several CT signs of gastrointestinal perforation have been described [2, 3]. Recently another CT finding related to colonic perforation called " dirty mass" has been reported [4]. We present a case of extraperitoneal rectal perforation secondary to colonoscopy in which CT demonstrated the presence of a focal collection of extraluminal fecal matter ("dirty mass") associated with pneumoretroperitoneum.