This is a report on ten patients with cystic dilatation of the intrahepatic biliary system (CDIB) after hepatic portoenterostomy. They were five girls and five boys and the diagnosis of CDIB was made at ages 6 months to 11 years (mean age: 2.8 +/- 3.3 years). Follow-up ranged from one month to 15 years (mean: 5.5 +/- 4.9 years). In order to elucidate the factors which affect the clinical outcome of such patients, the types of CDIB (Type A: noncommunicating solitary cyst, Type B: communicating solitary cyst, Type C: multi-cystic dilatation), clinical symptoms at onset of CDIB and the method for the treatment were reviewed in relation to the outcome. For the purpose of understanding pathogenesis of CDIB, immunohistochemical study on hepatobiliary system was done with monoclonal antibody for cytokeratin. Outcome of the patients of Type C was poor, whereas the outcome of patients with type A and B was good. The outcome of preoperatively jaundiced patients was poor, but jaundice-free patients showed good outcome. Method of treatment was not related to the outcome. As epithelium of CDIB was positive for monoclonal antibody of cytokeratin, it was suspected that pathogenesis of CDIB might be related to peribiliary gland which originated from ductal plate.