Woodchuck hepatocellular carcinoma has been successfully transplanted into nude (athymic) mice. The morphology of heterotransplanted tumor is similar to that of naturally occurring hepatocellular carcinoma before transplantation. The growth rate of transplanted tumor was very slow compared with those of other transplanted tumors. During the first month, only two tumors appeared. However, definitive tumor growth was noted in 6 of 20 nude mice about 3 months later. Seventeen of 20 nude mice exhibited sustained tumor growth after 6 months. The woodchuck hepatocellular carcinoma in nude mice provides an in vivo model for the study of oncogenesis of human hepatocellular carcinoma related to hepatitis B virus.