The number of hepatitis B virus (HBV) carrier babies has decreased markedly since the introduction in Japan of an "at-risk" strategy for preventing HBV infection. However, elimination of HBV infection from our country appears difficult. To clarify the limitations of the at-risk strategy for preventing vertical transmission of HBV, causes of vertical transmission in a single hospital were retrospectively analyzed. The following causes were presumed in 17 carrier pediatric cases: five patients had prenatal HBV infection, HBV infection during/after the immunization program was confirmed in five cases, two patients had prenatal infection or infection during the immunization program and three cases were caused by human error (by the patients' guardians). Because their mothers were HBV-negative at screening and only developed acute hepatitis B in the perinatal period, another two cases (Cases 3 and 10) did not undergo immunization because they were not subjects of the at-risk strategy. Sequence analyses in ten patients revealed genotype C (subgenotype, C2/Ce) in nine cases and genotype A (subgenotype, A2/Ae) in one case (Case 3). In Japan, HBV subgenotype Ae has recently been found more frequently among sexually active men with acute hepatitis. There are concerns that horizontal transmission of HBV from these men to their pregnant partners could increase. These data suggest clear limitations to the at-risk strategy in Japan and the possibility that the increase in genotype A may influence vertical transmission of HBV.