An autopsy report of a patient with a hugh biliary cystadenocarcinoma arising in the right lobe of the liver is presented. The tumor had been though to be benign, but peritoneal carcinomatosis developed 18 months after the operation. Autopsy revealed marked peritoneal dissemination of mucinous adenocarcinoma, apparently a result of peritoneal seeding of hepatic cystadenocarcinoma, which was presumed to be triggered by cyst aspiration performed during the operation. Experience indicates that surgical procedures on cystic liver disease should be carried out carefully, considering the possible existence of malignancy.