Gallstone ileus occurs when a large gallbladder stone erodes into the duodenum via a fistula, eventually obstructing the ileal lumen usually some centimeters proximal to the ileocecal junction.

Classical findings: small bowel obstruction, gas in the biliary tree and large ectopic gallstone (Rigler triad). Other findings: collapsed gallbladder, gas in the gallbladder.