PURPOSE: Choledochal cyst is a rare disease associated with an anomalous junction of the pancreaticobiliary ductal system (AJPBDS). We intended to demonstrate the characteristics of adult-onset choledochal cyst retrospectively. METHODS: ERCP findings of nineteen adult cases of choledochal cyst were compared with those of nine normal persons, in respect to the length and diameter of the common channel, and the angle of the pancreaticobiliary ductal junction. RESULTS: Patients of adult-onset choledochal cyst displayed a long, ectatic common channel and a larger angle of the pancreaticobiliary junction (p<0.05). All patients except one had AJPBDS. Thirteen patients were classified as Todani type Ia, one as type Ic and five as type IVa. Fourteen patients were classified as choledochopancreatic (C-P) type, two as pancreaticocholedochal (P-C) type, and three were unclassified by Kimura's classification. Sixteen patients were compatible with cystic type and three with cylindrical type by Okada's classification. Among the three cylindrical type patients two of them were classified as P-C type and one as C-P type. Histologically cystic type patients showed mild glandular or fibrotic patterns, however cylindrical type patients tended to show the severe glandular pattern. CONCLUSION: Adult-onset choledochal cyst was closely related with AJPBDS, although the reflux of pancreatic juice due to AJPBDS would not be the only cause to determine the morphology of adult-onset choledochal cyst. The meaning of this tendency toward a severe glandular pattern in cylindrical type patients requires elucidation.