Following conclusions were obtained on the basis of studies on endoscopic retrograde cholangiopancreatograms of 103 cases with diseases of the biliary tract. 1) Moderate to marked abnormalities of the pancreatic duct were observed in 65 percent of choledocholithiasis, whereas minimal to moderate abnormalities were found only in 28 percent of cholelithiasis. When changes of the pancreatic ducts with aging were taken into considerations, the corrected incidence was 57 percent and 17 percent respectively. 2) Moderate to marked abnormalities of the pancreatic duct were also frequently observed in cases of carcinoma of the Vaterian papilla and the common bile duct. 3) Most frequently observed abnormality was a dilatation of the pancreatic duct, followed by irregularity of the marigin, irregularity of the diameter and tortuosity of the pancreatic duct in the order described. 4) Diameter of the pancreatic duct was well corelated with that of the common bile duct. 5) Dilatation of the pancreatic duct and the common bile duct was well corelated with stenosis and irregularity of the Vaterian pancreatic and bile duct. 6) These data indicate that the following process is a main pathway of the pathogenesis of abnormality of the pancreatic duct in diseases of the biliary tract; diseases of the biliary tract→ abnormality of the Vaterian pancreatic and bile duct→ obstruction of outflow of pancreatic juice→ dilatation and other abnormalities of the pancreatic duct.