Gallstone ileus is an uncommon cause of bowel obstruction that involves cholecystoenteric fistulation and resultant passage of gallstones into the bowel. In the vast majority of cases, the fistula forms between the gallbladder and duodenum leading to small bowel obstruction. We report a case of cholecystocolic fistulation and subsequent large-bowel obstruction in a 75-year-old woman who presented acutely after taking a bowel preparation for an outpatient colonoscopy during the course of an investigation of anaemia and nonspecific abdominal pain. Preintervention imaging revealed a giant gallstone at the rectosigmoid junction, in the presence of a cholecystocolic fistula, and subsequent large bowel obstruction. After a failed period of expectant management, laparotomy and Hartmann's procedure were performed and the patient made an uneventful recovery.