The safety and immunogenicity of inactivated influenza virus vaccines in pregnant women have not been adequately investigated. In this study, 56 women received inactivated influenza A/New Jersey/76 virus vaccine during the second and third trimesters of pregnancy. No significant immediate reactions or increased fetal complications were associated with administration of the vaccine. The antibody response of the pregnant women to the vaccine was similar to that of nonpregnant adults. Forty mother-infant pairs were available for antibody surveillance. At delivery, reciprocal antibody titers of ⩾20 were present in 11 (42%) newborn (cord) sera and 15 (58%) maternal sera. Three months later, sera from only three infants (12%) contained this level of antibody. At six months, the serum of only one infant contained detectable antibodies. Levels of passively transferred antibodies from prior maternal infection with influenza A/Victoria/75 virus also declined rapidly following birth. It is possible that immunization of pregnant women can provide sufficient protection of the newborn infants by transfer of antibodies through the placenta if (1) a more potent influenza vaccine, possibly used with booster dosing, is administered, and (2) the women deliver just prior to or during the influenza season.