A big gallstone penetrating through a bilio-digestive fistula into the bowel can make a complete obstruction of it in the majority of the cases requiring an urgent operation. It is a rare situation that a fistula like this leads into the colon and the stone makes a large bowel obstruction. A 72-year-old male patient was admitted to our department with the symptoms of colon obstruction. After the examinations he underwent an explorative laparotomy. During the course of the operation an obstruction was found in the proximal third of the descending colon. A subtotal colectomy was carried out. At the opening of the resected colon a big gallstone was found as the cause of the obstruction, which simulated a tumorous obstruction. We present through this case the possibilities and results of the diagnosis and treatment of gallstone ileus. We conclude that in the prevention of gallstone ileus, the early operative treatment of detected gallstone is essential.