Background: The purpose of this study was to develop a new concept of the embryonic etiology of pancreaticobiliary maljunction (PBM) based on cholangiopancreatograms. Methods: The subjects were 202 patients with PBM (60 men and 142 women) in whom the junction of the pancreatic and bile ducts was radiologically diagnosed as being located outside of the duodenal wall; 133 of the 202 patients also had congenital cystic dilatation of the bile duct (CCBD). Results: The length of the duct from the junction to the orifice of the major papilla (the common channel) ranged from 0.5 to 5 cm on the cholangiopancreatograms. Small radicles of the pancreatic duct arose from the common channel in 36 of the 202 patients. This finding suggests that the common channel is itself the main pancreatic duct in patients with PBM. Moreover, cholangiopancreatography revealed that in 99 of the 202 patients, there was a narrowed duct segment distal to the biliary cyst in patients with CCBD or distal to the normal bile duct in those without CCBD; the length of the narrowed segment varied. Histologic examination revealed smaller branches that had arisen from this narrowed segment in 2 anatomic specimens. This also suggests that the narrowed ductal segment belongs to the pancreatic duct system. Conclusion: PBM is an anomaly that is probably caused by a disturbance in the embryologic connections (misarrangement) of the pancreatic and biliary duct system that occurs extremely early during gestation when the bile duct joins with the ventral pancreatic duct system. PBM is not due to an arrest of the normal migration of the common channel into the duodenal lumen during embryonic development. (Gastrointest Endosc 2001;53:614-9.)