The infiltration of extrahepatic bile duct carcinoma along the bile duct wall was studied in 29 resection cases. Histologically, spreading took place by mucosal infiltration in one case, mucosal and transmural in three, and transmural in 25. The average microscopic infiltrating distance from the gross margin of the tumor (ID) was 16.8 mm towards the liver, 6.5 mm towards the duodenum. The ID of the infiltrating type in the gross appearance of the tumor and the ID of the tubular adenocarcinoma in the histological cellular type tended to be long. The severer the invasion of the carcinoma to the lymphatic vessels, the longer the ID. The result indicated that most bile duct carcinomas have a tendency for long infiltration along the wall. An exceedingly wide resection involving hepatectomy is advisable for treatment.