A 78-year-old Japanese man with undifferentiated carcinoma of the common bile duct is presented. Upon gross examination, the common bile duct was found to be obstructed by a nodule measuring 10 x 10 mm. Microscopically, the nodule was ill-defined and composed of atypical spindle-shaped and pleomorphic tumor cells. The spindle-shaped cells proliferated in a whirled or interlacing pattern simulating a sarcoma, and the pleomorphic tumor cells had abundant eosinophilic cytoplasm and bizarre nuclei. Histochemically, a few tumor cells contained mucosubstances stained with the alcian blue (AB) method in their cytoplasm. Immunohistochemically, the tumor cells were diffusely positive for CAM5.2 and AE1/AE3. The histological diagnosis was undifferentiated carcinoma (spindle cell carcinoma) of the common bile duct. Other than our patient, only four other cases of undifferentiated carcinoma in the extrahepatic bile duct have been reported in the literature.