Eight cases of choledochal cyst associated with biliary malignancy (gallbladder carcinoma in three and bile duct carcinoma in five) were reviewed to evaluate the roles and limitations of computed tomography (CT) (n = 8), ultrasound (US) (n = 6), cholangiography (n = 8), and angiography (n = 6). In cases of gallbladder carcinoma, both CT and US revealed mass lesions in the gallbladder consistent with cancer. CT also depicted either a mass lesion or an irregular thickened wall in all cases of bile duct carcinoma; however, US failed to demonstrate a thickened wall in one of three cases in which CT findings were positive. Cholangiography universally revealed malunion of the pancreatico-biliary duct, and the findings were suggestive of malignancy in seven cases in which CT depicted abnormalities. Angiography showed tumor stain in three of six cases. Lymph node metastases were present in four cases but were detected preoperatively in only one. One case showing a thickened bile duct wall was erroneously diagnosed as malignancy among 27 cases of choledochal cyst unassociated with biliary malignancy.