An adenocarcinoma was found in a choledochal cyst in a 12-year-old girl. This is the youngest patient so far reported with an adenocarcinoma in a choledochal cyst. The cyst consisted of dilatations of the intrahepatic bile ducts and the extrahepatic bile duct. Most of the cyst wall was resected. However, resection of the distal part of the cyst wall in the infrapancreatic region was impossible because of invasion of the portal vein and the inferior vena cava. This patient is now being treated with chemotherapy. We believe that early diagnosis is mandatory, and that total excision of a choledochal cyst is advisably to prevent the development of carcinoma.