A prospective study was undertaken in Jerusalem to quantitate the HBsAg prevalence rate in pregnant women, to assess the effect of ethnic origin on HBsAg prevalence rates, and to determine the impact of HBsAg carrier state on hepatitis B virus horizontal transmission in the family. Of the 6,572 women screened at the time of delivery, 42 (0.64%) were found to be HBsAg positive. Although the percentage of Moslem women was 3.4% of the total study population, they accounted for 23.8% of the HBsAg positive mothers. This resulted in a prevalence rate of 4.48% as compared to only 0.5% in the Jewish population (P < 0.01). Nine percent of siblings born to HBsAg positive mothers and 29% of the husbands were found to be horizontally infected.