The objective of this study was to investigate household clustering of hepatitis B virus (HBV) infection in South Africa in order to understand intra-household patterns of virus transmission that would provide information on potential risk factors of HBV infection. Subjects were the household contacts of 28 hepatitis B surface antigen (HBsAg) positive children (index-carrier), 22 hepatitis B surface antibody positive children (index-past-infection) and 35 children with no serological evidence of HBV infection (index-negative). Evidence of HBV infection (at least one positive HBV marker) was present in 73.7%, 48.7% and 38.2% and HBsAg was present in 19.9%, 8.7% and 2.9% of household contacts of index-carrier (N = 186), index-past-infection (N = 150) and index-negative (N = 207) children respectively. The clustering of HBV infection and HBsAg was present in all subgroups of household contacts regardless of the degree of relatedness to the index child. As age increased, the cumulative prevalence of HBV infection increased while the likelihood of being HBsAg positive decreased. Regardless of age, males were more likely (p less than 0.01) than females to be HBsAg positive. We conclude that HBV infection clusters in households; that there is an increased susceptibility, which is probably not genetically mediated, of becoming an HBV carrier in certain households and that males are at greater risk of being HBV carriers. Intra-household horizontal person-to-person transmission, the precise mechanisms of which are not known, is important in South African blacks.