Although it has been hypothesized that congenital dilataion of the common bile duct is caused by the reflux of pancreatic juice into the choledochus, the etiology of this disease is not yet established. To determine whether bile acids have relations with the cause of congenital biliary dilatation or haven't, we analyzed the choledochal bile in a patient with congenital biliary dilatation. The results showed that the level of lithocholic acid, which has been said to cause bile-stasis was within normal range and that unknown bile acids were not detected in this patient. Therefore, it has been con eluded that bile acids have no relations with the etiology of congenital biliary dilatation.