The study was carried out to observe the effects of Clonorchis sinensis infection on induction of cholangiocarcinoma in Syrian golden hamsters to which 15 ppm dimethylnitrosamine (DMN) solution was administered for 8 weeks. The histopathological changes of the bile duct and liver cells were observed at the 11th week. In six of 8 hamsters (75%) which were treated with DMN and then infected with C. sinensis, the livers developed cholangiocarcinoma at 10 weeks after the infestation of C. sinensis. The features of cholangiocarcinoma lesions were adenomatous or papillary hyperplasia of the bile duct epithelia showing distinct anaplastic changes with mucinous cell metaplasia and necrotic area. In the hamsters which received either DMN or C. sinensis alone, the livers showed only hyperplastic changes of the bile duct epithelial cells. It was suggested that C. sinensis infection and DMN administration could be a synergism on the development of cholangiocarcinoma in Syrian golden hamsters.