Endoscopic retrograde choledochopancreatography entails passing a special endoscopic tube system into the duodenum, entering the pancreatic duct or the common bile duct with the end of a cannula, and injecting x-ray contrast material. The procedure is used predominantly for diagnosis of pancreatic disease, but it may occasionally be helpful in equivocal cases of biliary tract obstruction. Cannulating the common bile duct is not easy, and best results are obtained by very experienced endoscopists.