Pancreaticobiliary maljunction (PBM) is a congenital anomaly defined as a junction of the pancreatic and bile ducts located outside the duodenal wall, usually forming a markedly long common channel. In PBM patients, this anomaly allows regurgitation between the pancreatobiliary and biliopancreatic tract. Since hydrostatic pressure within the pancreatic duct is usually higher than that in the common bile duct, pancreatic juice frequently refluxes into the bile duct. As a result, pancreatic enzyme levels are generally very high in the bile and there is a related high incidence of biliary cancer. PBM can be divided into PBM with biliary dilatation (congenital choledochal cyst [CCC]) and PBM without biliary dilatation (maximal diameter of the bile duct <or= 10 mm). In 42 CCC patients, bile duct and gallbladder cancer was seen in 7 and in 8, respectively. In 49 PBM patients without biliary dilatation, only gallbladder cancer was detected in 33 patients. Because of the increased cell proliferation with random K-ras mutations, the gallbladder mucosa of PBM patients should be considered premalignant. CCC treatment is prophylactic flow-diversion surgery, but only prophylactic cholecystectomy is usually performed for PBM without biliary dilatation. Pancreatobiliary reflux and premalignant changes in the gallbladder can occur in patients with a relatively long common channel (high confluence of pancreaticobiliary ducts).