Bile duct carcinoma, which produces clinically recognizable mucus, was defined as "mucus producing bile duct carcinoma", and clinicopathological study was carried out in 7 cases of bile duct carcinoma suitable for the definition. All the tumors arose from the intrahepatic bile duct. There were no tumors arising from the extrahepatic bile duct. Superficially spreading mucosal infiltration of carcinoma was recognized in 6 cases out of 7, and accordingly distinct cholangiography after draining mucus through percutaneous transhepatic cholangio-drainage (PTCD) and percutaneous transhepatic cholangioscopy (PTCS) were indispensable for accurate diagnosis of the extent of carcinoma. The prognosis of patients with mucus producing bile duct carcinoma were almost satisfactory if rational operation had been performed according to accurate diagnosis. On the other hand, since mucus producing bile duct carcinoma frequently has a cystic lesion, the relation to biliary cystadenocarcinoma may become a subject of question. We advocate that biliary cystadenocarcinoma should be included in mucus producing bile duct carcinoma since biliary cystadenocarcinoma originally arises from the intrahepatic bile duct and very rarely from the extrahepatic bile duct. But now the concept of biliary cystadenocarcinoma is equivocal and further investigations will be requested.