증례보고의 목적 및 강조 사항: Choledochal cyst is a rare disease; nonetheless, it has a clinical importance because of high incidence of biliary tract cancer, even in asymptomatic patient. The importance of complete cyst excision is commonly understood to prevent the development of malignancies and other complications. In this case, common bile duct cancer arising from remnant choledochal cyst, 9 years after excision of a choledochal cyst and it manifested as mucinous cystic neoplasm of the pancreas. To our knowledge, this is the first reported case in the English-language literature. 초록본문: We report a rare case of common bile duct cancer arising from remnant choledochal cyst which was incompletely excised 9 years ago, manifesting mucinous cystic neoplasm of the pancreas, respectively. A 42-years-old woman was admitted to our hospital because of dyspepsia and abdominal discomfort for 1 week. She had undergone operation for cholecochal cyst with gall bladder cancer 9 years ago; radical cholecystectomy including lymph node dissection and liver wedge resection, choledochal cyst excision, Roux-en-Y hepaticojejunostomy, distal pancreatectomy and splenectomy for multinodular hard mass in tail of pancreas. Pathology revealed a polypoid mass in gall bladder with liver infiltration as poorly differentiated adenocarcinoma, and a pancreatic tail mass as chronic pancreatitis. Based on her past medical history, we conducted imaging studies including CT, MRCP, EUS. Imaging showed a newly developed suspected solid nodule in the peripheral portion of cystic lesion in the pancreas head. She underwent a pylorus preserving pancreaticoduodenectomy for the suspected mucinous cystic neoplasm of the pancreas. At surgery, two hard mass were notified in the dilated remnant intrapancreatic choledochal cyst. Pathology revealed common bile duct cancer as poorly differentiated adenocarcinoma. The remnant choledochal cyst had developed to malignancy. Thus, complete excision is a crucial treatment strategy of choledochal cyst.