This case report describes a 77-year-old male, who presented to the emergency room with symptoms of an acute proximal small bowel obstruction. Abdominal CT scan with multi-planar reconstructions led to the diagnosis of an intestinal obstruction due to impaction of a large gallstone in the second portion of the duodenum. The CT scan demonstrated a large cholecysto-duodenal fistula as the origin of the gallstone migration. Surgical treatment consisted of milking the stone down beyond the ligament of Treitz, where it was removed through a jejunal enterotomy. The postoperative course was uncomplicated. No attempt was made to repair the choledocho-duodenal fistula at the initial intervention nor subsequently, and there have been no complications due to the fistula over 36 months of follow-up observation.