Gallstone ileus is a rare form of small bowel obstruction caused by an impaction of a gallstone within the lumen of the small intestine. Such a gallstone is hypothesized to enter the bowel via a cholecysto-enteric fistula. The presence of large stones, >2.5 cm in diameter, within the gallbladder are thought to predispose to fistula formation by gradual erosion through the gallbladder fundus. Once a fistula has formed, a stone may travel from the gallbladder into the bowel and become lodged almost anywhere along the GI tract. Obstruction occurs most commonly at the near the distal ileum, within 60 cm proximally to the ileocecal valve.