Choledochoduodenal fistula is an uncommon complication of choledocholithiasis. Five patients with this entity were treated by endoscopic enlargement of the choledochoduodenal fistula without complication. Choledochoduodenal fistula complicating choledocholithiasis seem to arise either from stone erosion through the bile duct into the duodenum or as a complication of vigorous instrumentation at the time of common duct exploration. In patients in whom endoscopic sphincterotomy cannot be performed because of inability to cannulate the ampullary orifice, endoscopic enlargement of the choledochoduodenal fistula can be performed safely.