Abstract Thirteen patients undergoing investigation for biliary or pancreatic disease were found at endoscopic retrograde cholangiopancreatography to have unsuspected choledochoduodenal fistula. In only one patient could the presenting symptoms be directly attributed to the fistula. Ten of the thirteen patients had previously undergone exploration of the common bile duct and instrumentation at surgery is postulated as being the most likely cause of these fistulae. Treatment of the uncomplicated fistula appears unnecessary.