An HBV carrier rate of 3.0% in outpatients and 1.4% in school children, was found in a sero-epidemiological survey in Muranga District, Kenya. The prevalence of anti-Delta in HBV carriers was 42%. The prevalence of HBV carriers and HBV marker positive cases was high in family members of HBV carriers. The yearly attack ratio of HBV was low in primary school children, non-carrier family members and even carrier family members. Mother to baby vertical transmission was very high when mothers were HBeAg positive HBV carriers. Such vertical transmission may now be of major importance.