AIM: To clarify the pathogenesis of biliary dilatation associated with anomalous union of the pancreatic and biliary ducts (AUPBD).METHODS: Direct cholangiopancreatograms of 350 adult patients with or with suspicion of hepatobiliary or pancreatic disorders were reviewed. AUPBD was diagnosed cholangiopancreatographically, when the pancreaticobiliary ductal union was located above the narrow distal segment of the bile duct, which represents the action of the sphincter of Oddi. The narrow portion of the terminal choledochus was defined as symmetrical stricture of the common bile duct just above the pancreaticobiliary ductal union.RESULTS: AUPBD was found in 36 patients. Among cholangiopancreatographic features, the narrow portion of the terminal choledochus was the most pathognomonic for AUPBD (accuracy, 98%); it was present in 29 (81%)patients with AUPBD, but was not found in any patients without AUPBD. Among patients with AUPBD, biliary dilatation (＞10 mm) was more frequent in those with the narrow portion of the terminal choledochus (23/29)than in those without (2/7; P = 0.018) AUPBD. Among the patients with both AUPBD and the narrow portion of the terminal choledochus, there was a strong negative correlation between the minimum diameter of the narrow portion and the maximum diameter of the choledochus (r = -0.78, P＜0.001), suggesting that the degree of biliary narrowing at the narrow portion correlates with that of upstream biliary dilatation.CONCLUSION: The narrow portion of the terminal choledochus, a pathognomonic radiologic feature of AUPBD, may be a cause of biliary dilatation in patients with AUPBD.