Between 1960 and 1975, 17 patients with congenital cystic dilatation of the common bile duct (choledochal cyst) were treated and three were associated with malignant tumors in the cysts and one was with carcinoma of the gallbladder. Preoperative diagnosis of adenocarcinoma in the choledochal cyst was established in one patient by cytologic examination of the bile which was obtained during the procedure of endoscopic pancreaticocholangraphy (EPCG) and percutaneous transhepatic cholangiography (PTC). Definitive treatment of the choledochal cysts associated with malignant tumors in the biliary tract was accomplished by excision of the cysts with tumor and choledochojejunostomy in two patients, by cystoduodenostomy following external drainage in one and by cholecystectomy with resection of invaded transverse colon in one with cancer of the gallbladder. Successful excision of choledochal cysts in 11 patients including three cases associated with malignant tumors in the biliary tract during the last 15 year period is the basis of this report.