The role of the hepatitis B virus (HBV) X protein in liver tumorigenesis is unresolved. Transgenic mice harboring the X gene (nt 1376–1840 under the control of the human α‐1‐antitrypsin regulatory elements) (ATX mice) display only minor histopathologic alterations of the liver. To determine if ATX mice are more susceptible to the effects of hepatocarcinogens, 12‐ to 15‐d‐old male ATX and control littermate mice were injected with a single dose (2 μg/g body weight) of diethylnitrosamine (DEN). The animals were killed 6–10 mo after exposure and were analyzed for histological changes in the liver. One hundred percent of the DEN‐treated ATX mice developed abnormal liver lesions. When their liver tissues were compared by stereological analysis with those of non‐transgenic animals, the ATX mice had a relative twofold increase in the total number of focal lesions and a twofold increase in the incidence of hepatocellular carcinoma. Elevated levels of X protein and p53 protein were not detected in carcinogen‐induced nodules or tumors. These results are consistent with a model in which the expression of the HBV X protein potentiates the induction of DEN‐mediated liver disease. © 1996 Wiley‐Liss, Inc.