Epidemiologic observations show a higher frequency of hepatitis B virus (HBV) serologic markers in chronic alcoholics compared with the general population. This may be the result of an increased susceptibility of alcoholics to infection and/or to an ethanol-mediated stimulation of HBV gene expression and replication. To test the latter hypothesis, HBV transgenic SCID mice, which support consistent levels of virus replication, were fed with a standard Lieber-DiCarli or isocaloric diet for 5 weeks. In ethanol-fed mice, the levels of hepatitis B surface antigen (HBsAg) and viral DNA in serum increased by up to 7-fold compared with mice fed the control diet. Ethanol-treated mice also had elevated HBV-RNA levels, and increased expression of surface, core, and X antigens in the liver, especially in the pericentral regions. None of these changes were observed in transgenic mice fed isocaloric diets. Thus, chronic alcohol consumption alters the patterns of HBV gene expression and replication in the serum and liver of HBV transgenic SCID mice, and may provide a partial explanation for the increased frequency of HBV markers among alcoholics. (HEPATOLOGY 2001; 34: 792-797.)