This is a retrospective study of 13 patients with sclerosis of the common bile duct seen over a ten-year period at the Hospital of the Good Samaritan Medical Center. Histologic examination of the common bile duct in eight patients permitted a clear-cut diagnosis of sclerosing carcinoma. In the remaining five patients, glandular atypia and sclerosis were suggestive, but not diagnostic, of carcinoma. Three of these were eventually established as carcinoma at autopsy. The diagnosis in the last two patients, who are still alive, remains equivocal. This study emphasizes the difficulty of separating benign from malignant disease of the common bile duct and suggests that the majority of patients diagnosed as having primary sclerosing cholangitis will eventually be proved to have carcinoma. It further identifies the protracted course that some of these patients exhibit.