An 81-year-old man presented with abdominal pain and repeated bilious vomiting for 1 day. He had cholelithiasis for 5 years, documented by CT (A). This was confirmed on the lowest slice of a thoracic CT performed 6 months earlier for an unrelated pulmonary condition (B). Note the migration of the gallstone from the fundus of the gallbladder to the infundibulum. After admission, a CT demonstrated a fluid-filled gallbladder with a small amount of air and the absence of cholelithiasis (C). The duodenum was totally obstructed at the fourth portion by a calcified mass (D). At operation, a single gallstone measuring 5 5 3 cm (E) was present just distal to the ligament of Treitz. It was maneuvered distally, and removed by a proximal enterotomy. Based on the recent experience of others, no attempt was made to remove the gallbladder or close the cholecystoduodenal fistula. Recovery was uneventful. Cholecystoduodenal fistula commonly develops from the pressure and irritation of a large gallstone eroding through the adherent walls of the gallbladder and the adjacent duodenum, allowing the gallstone to enter the gastrointestinal tract and obstruct it. These images, obtained over a 5-year period, document the migration of the gallstone from the fundus of the gallbladder to the infundibulum and, ultimately, into the lumen of the duodenum, documenting the progression of disease. The most common site of intestinal obstruction is at the distal ileum where the lumen normally narrows, but it can occur anywhere along the gastrointestinal tract. Because of the size of this gallstone and the natural angulation caused by the attachment of the ligament of Treitz, a high obstruction resulted and accounted for the frequent bilious emesis seen in this patient.