Opisthorchis and Clonorchis parasitize the bile ducts of millions of persons in the Far East. The most important aspect of infection with these flukes is their ability to initiate cancer. Numerous studies have shown these flukes to occur in association with cholangiocarcinoma far more frequently than can be explained by chance. Experimental studies in animals have confirmed the carcinogenic potential of these parasites. Although the pathogenesis remains unclear, the initial carcinogenic event is probably a function of the length and severity of infection, the host's immune response, and such variables as ingestion of dietary carcinogens.