PURPOSE: To review the type, incidence and radiologic findings and to evaluate the clinical significance of intrahepatic duct dilatation of choledochal cysts in children. MATERIALS AND METHODS: Ultrasonography and operative cholangiography of 49 consecutive children with surgically confirmed choledochal cyst were reviewed and classified according to the Todani's classification. Patients with intrahepatic duct dilatation in preoperative evaluation were routinely followed by ultrasonography after surgery of extrahepatic bile duct. The status of bileduct was evaluated by comparing with the preoperative ultrasonography. RESULTS: According to the preoperative evaluation, there were 19 cases(39%) of type Ia, three cases(6%) of type Ib, ten cases(20%) of type Ic, 15 cases(31%) of type IVa, two cases(4%) of type V, but no case of the type II, III, IVb in Todani's classification.In twelve of 13 patients (five cases of type I and eight cases of type IVa) who had been followed up after surgery of choledochal cyst, the previously noted intrahepatic ductal dilatation were no longer seen. CONCLUSION: The incidence of the types of choledochal cysts by Todani's classification shows no significant difference between our result and the other authors'. The intrahepatic ductal dilatation seems to be reversible and mainly secondary to extrahepatic obstruction by choledochal cyst. Thus, postoperative follow up examination is more important than preoperative classification of bile duct dilatation.