Choledochal cyst is a congenital dilatation of the bile duct. Intrahepatic bile duct dilatation of type IVa by Todani's classification at the time of diagnosis resolved spontaneously after cyst excision and hepaticojejunostomy in many cases. It should be distinguished from the true cystic dilatation of the intrahepatic ducts, which tends to persist, albeit after some regression. We therefore studied postoperative intrahepatic duct dilatation changes in choledochal cyst. A total of seventy-six choledochal cysts were managed at the Division of Pediatric Surgery, Department of Surgery, Samsung Medical Center from May 1995 to December 2005. The ratio of males to females was 1:2.8. Preoperative radiologic diagnosis by Todani's classification was Type I (n=52, 68.4 %), II (n=1, 1.3 %), IVa (n=23, 30.3 %). Among fifty-five patients with intrahepatic bile duct dilatation we were able to follow up forty-eight by ultrasonography. Twenty-two patients were type IVa, and twenty-six patients were type I and showed intrahepatic duct dilatation. Mean follow-up duration was 35.3 months (9~105 months). Complete regression of dilated intrahepatic duct was observed in fifteen patients of type IVa and twenty-four patients of type I. Incomplete regression of dilated intrahepatic duct was observed in six patients in type IVa and two patients in type I. Only one patient in type IVa showed no change in ductal dilatation during a follow-up period of 15 months. We conclude that true type IVa is much less frequent than what was diagnosed preoperatively by imaging study. Therefore in type IVa patients who are diagnosed preoperatively the decision to perform liver resection should be carefully considered. Postoperative long term follow up of choledochal cyst with intrahepatic bile duct dilation is needed.