Improvement in prenatal ultrasonography is leading to diagnose choledochal cyst before birth and before onset of classical symptom more frequently. But, there is a controversy about optimal timing for Cyst excision of prenatally diagnosed asymptomatic choledochal cyst. To identify the most appropriate timing for surgery in prenatally diagnosed choledochal Cysts, we analyzed 6 patients who had operation for choledochal cysts within 30days after birth at the division of Pediatric Surgery, Samsung Medical Center and Inha University School of Medicine, from June 1995 to June 2002. Males were four and females 2, the mean age at operation was 11.2 days, and the median age 8.0 days. The range of gestational ages of the antenatal diagnosis of bile duct dilatation was 24 weeks to 32 weeks, mean was 38.3 weeks, and mean birth weight was 3,298.3 g. After birth, abdominal ultrasonography, hepatobiliary scintigraphy, and magnetic resonance cholangiopancratography (MRCP) were performed. Mean age at operation was 11.2 days. All patients had the cyst excision and Rouxen-Y hepaticojejunostomy. Immediate postoperative complication was not found. During the median follow-up period of 41 months, one patient was admitted due to cholangitis, and the other due to variceal bleeding. Early operative treatment of asymptomatic newborn is safe and effective to prevent developing complications later in life.