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 gut lumen via cholecysto-enteric fistula. Large stones, >2.5 cm in diameter, are thought to predispose to fistula formation by gradual erosion through the gallbladder fundus. Most commonly, obstruction occurs at the distal ileum (i.e. ileocecal valve), but can occur at almost any other location in the GI tract.Classically there is obstruction/impaction about 60 cm proximal to ileocaecal valve.