Swine influenza virus passive antibody levels were measured in pigs from vaccinated vs. nonvaccinated dams. Pigs from vaccinated dams had significantly higher passive antibody titers compared with pigs from nonvaccinated dams. In pigs from nonvaccinated sows, titers >20 were not detected by 8 weeks of age. In pigs from vaccinated dams, titers >20 were no longer detected at 16 weeks of age. The persistence of passive antibodies may protect young pigs from SIV but also could inhibit immune responses to vaccination.