In view of the benefits of immunoprophylaxis among newborns of mothers who are seropositive for hepatitis B virus (HBV) surface antigen (HBsAG), these women must be correctly identified so that this measure can be instituted in Brazil. The research reported here studied 7992 women who gave birth in the Hospital das Clínicas of the Ribeirão Preto School of Medicine, University of São Paulo (HCFMRP-USP), Brazil, in order to determine the rate of serum reactivity of HBsAg and other markers of HBV infection among these women, and also to evaluate the risk factors for this infection. Serum reactivity for HBsAg was determined by means of an immunoenzymatic test (ELISA) carried out in two stages: the first with an incubation period of 2 hours (screening), and the second with an incubation period of 18 hours (confirmation) for those samples that were positive in the screening test. The markers anti-HBsAg, HBeAg, anti-HBcAg, and anti-HBeAG were tested in the samples that were confirmed positive. The screening test found 1.05% (95% CI: 0.84 - 1.30) of the samples to be positive for HBsAg. However, only 0.95% (95% CI: 0.75 - 1.19) were confirmed positive, the percentage being significantly higher among patients whose pregnancies had ended in abortion (1.84%) than among those who had given birth (0.84%) (X2, Yates correction = 7.76; P < 0.005). Risk factors for HBV infection could be identified for only 27.6% of the study subjects, based on their recall and reporting. Of the women with confirmed positive samples, 21.3% also were positive for HBeAG, indicating that these patients ran a greater risk of transmitting the virus vertically. These results underline the need for specific serologic studies in the final stage of pregnancy in order to offer the maximum benefit of neonatal immunoprophylaxis.