Transmission of Hepatitis B virus from mother to infant can cause a severe type of neonatal hepatitis. Comparing chronic carrier mothers with mothers who have acute hepatitis in pregnancy we find that the latter group present the greater risk to their infants. Differences in the presence of Hepatitis B core and Hepatitis B e antigen and antibody in the acute case and in the carrier state may expoain the differences observed in the transmission rates of two groups. In countries where the prevalence of the carrier state is high it appears that vertical transmission of the carrier state is also common. This pattern may be explained by differences in the geographical distribution of carriers of Hep B e antigen, which has been recently described as a virulance factor.