A 17 years old boy was admitted into our hospital with symptoms suggesting ileus. While the native X-ray imaging further supported the diagnosis of ileus, the ultrasound examination showed a cystic mass on the distal third of the colon descendens; furthermore, also raised the possibility of appendicitis. To clarify the situation, the contrast CT revealed an intraluminar fluid-filled mass in the sigma. The accurate and focused questions of the surgeon aided us with the final diagnosis: our patient has placed an inflated balloon into the rectum. Surgical intervention was not necessary. The foreign body was successfully removed by endoscopic way.