The aim of this study was to clarify the appropriate management after birth for congenital biliary dilatation (CBD, choledochal cyst) patients with a prenatal diagnosis. Thirteen patients with a prenatal diagnosis of CBD who underwent liver biopsy during excision surgery were divided into two groups and retrospectively analyzed: group A, with liver fibrosis above F1 and group B, without liver fibrosis. Excision surgery was performed earlier in group A (F1-F2), at a median of 106 days old (p=0.04). There were significant differences between the two groups in the presence symptoms and sludge, the cyst size, and the level of serum bilirubin and gamma glutamyl transpeptidase (GGT) before excision surgery (p<0.05). Especially, in group A, prolonged serum GGT elevation and larger cysts were consistently observed from birth. The cut-off values of predictions for the presence of liver fibrosis in serum GGT and cyst size were 319U/l and 45mm. No significant differences were observed in the postoperative liver function or complications during the follow-up period. In patients with prenatally diagnosed CBD, the postnatal serial changes of serum GGT values and cyst size, in addition to symptoms, could help to prevent progressive liver fibrosis. Ⅲ. Treatment Study.