We analyzed the relationship between the presence of intrafamilial clustering of infection with hepatitis B virus (HBV) and the condition of the liver. Parents and siblings of subjects infected with HBV, some patients and some carriers, were tested for the presence of HBsAg, HBeAg, HBeAb, and liver damage. Then the original subjects with HBV were classified by the results into one of three groups. The subjects in the group without clustering had a higher rate of being seronegative for HBeAg than the groups with clustering, at all ages (P less than 0.01; test). Of the original subjects who were seronegative for HBeAg, the group without clustering had less damage of the liver than the groups with clustering. Of the subjects infected by horizontal transmission, the group without clustering had a higher rate of being seronegative for HBeAg and less damage of the liver than the groups with clustering. Subjects with HBV in a family with members who had normal liver function and who were seronegative for HBeAg were less likely to develop chronic hepatitis B than such subjects in a family with members having chronic liver damage. Thus the presence of intrafamilial clustering might affect the chance of subjects with HBV developing liver damage. The mode of infection and some genetic factors in the infected subjects seem to contribute to the condition of subjects with HBV infection.