Liver biopsy specimens from a primary biliary cirrhosis (PBG) patient and two control patients with other biliary disorders were examined under the electron microscope, with special reference to intrahepatic bile ducts. The PBG patient had a bile duct with an 80 μm diameter that showed specific alterations in epithelial cells. These cells were termed oncocytes, and their main features were: the presence of a large number of swollen mitochondria with abnormal cristae, no mitochondrial bodies, small number of exocrine granules and an undulated nucleus with a large cytoplasm. The number of mitochondria per ultrathin‐sectioned epithelial cell was greater by about 2 times in the oncocytic epithelial cells compared to the control cells. Fine fibrils and long‐spacing collagens were present around the PBC bile duct. Periductal capillaries of the bile duct had multilayered basement membranes. These periductal changes may have induced the oncocytic changes in the epithelial cells. The basement membrane of the bile duct was meandrous and often disrupted, but not multilayered. Many lymphocytes and macrophages infiltrated among the epithelial c