Congenital dilatation of the bile duct (CDBD) or choledochal cyst has been demonstrated to be associated with an anomalous junction of the pancreaticobiliary ductal system. Multifarious clinical signs and symptoms of CDBD have been shown to be closely related with the presence of this anomalous junction. In the present study, 100 instances of CDBD treated surgically at our institutions during a 30 year period were classified into two types according to the morphologic features of dilatation of the bile duct; there were 77 instances of the cystic type and 23 of the cylindric type. Morphologic features of the lesion, clinical signs and symptoms and laboratory findings in these 100 instances were clinically analyzed. In almost all of the patients who were less than one year of age, the disease was of the cystic type and patients presented with either a palpable mass or jaundice as the main symptom. In patients more than one year of age, the disease was of either the cystic or cylindric type. A history of episodes of characteristic abdominal pain accompanied by elevated levels of serum amylase was present in 70 of the patients with the cystic type of disease and in all of the patients with the cylindric type. Histologic sections from the patients showed glandular formation with chronic inflammation, possibly a result of refluxed activated pancreatic juice; in contrast, histologic sections from the remaining patients of all ages showed only thickening of the fibrous layer. Thus, such variable morphologic features and clinical signs and symptoms in CDBD are highly dependent on two factors--the age at onset and the reflux of pancreatic juice into the bile duct through the common channel.