Objective: To determine the clinical features and clinical outcomes of Korean adults treated surgically for choledochal cyst. Design: Retrospective nationwide multicenter study. Setting: Fifteen university hospitals (tertiary care referral centers) located in all 7 Korean provinces. Patients: A total of 808 patients aged 18 years or older who underwent surgery for choledochal cyst from January 1, 1990, through December 31, 2007. Main Outcome Measures: Demographic information, surgical data, associated biliary malignant tumors, and factors predicting malignant tumors. Results: Type I was most common (499 [68.2%]) followed by type IVa (208 [28.4%]). Of 654 patients, anomalous pancreaticobiliary ductal union was identified in 467 patients (71.4%), 291 with the choledochal type (62.3%), 96 with the pancreatic type (20.6%), and 80 with the complextype (17.1%). Biliary tract malignant tumor was associated in 80 patients (9.9%); 40 had bile duct cancer (50.0%), 35 had gallbladder cancer (43.8%), 3 had periampullary cancer, and 2 had synchronous gallbladder and bile duct cancer. Twenty-two patients (26.3%) had a recurrence, with a median follow-up duration of 51.8 months. Factors predicting malignant tumor by univariate analysis were age more than 40 years, the absence of a gallstone, elevated carcinoembryonic antigen or cancer antigen 19-9 serum level, and the presence of anomalous pancreaticobiliary ductal union, and by multivariate analysis, an elevated cancer antigen 19-9 level. Conclusions: Associated biliary malignant tumor should always be considered in patients with choledochal cyst, especially in aged patients or patients with anomalous pancreaticobiliary ductal union or an elevated tumor marker level. Lifelong follow-up is needed even after complete cyst excision because of the risk of the development of a metachronous biliary malignant tumor.