To monitor the development of specific and cross-reactive antibody response in twenty Japanese volunteers after vaccination with live yellow fever vaccine. Serum samples were collected on various days after vaccination and examined for hemagglutination inhibition (HI) antibodies against yellow fever virus (YFV), Japanese encephalitis virus (JEV) and dengue virus (DV), neutralizing antibodies against YFV and JEV, and IgM antibodies against YFV. None of the volunteers had been previously immunized with this vaccine. Fifteen of 20 had pre-vaccinated with JEV 7 to 40 years before. Ten of the 20 had neutralizing antibodies against JEV before immunization. None of the 20 had detectable antibodies against YFV or DV before vaccination. On day 10th after the vaccination, neutralizing antibodies to YFV were detected in 6 of 19 volunteers and IgM antibodies against YFV were detected in 7 of 19. On day 14th, HI, neutralizing, and IgM antibodies against YFV were detected in all the tested sera. Neutralizing antibodies against JEV were developed in 2 volunteers and HI antibodies against JEV were increased in 3 of 6 volunteers respectively. On day 29th, cross-reactive HI antibodies for JEV and DV were detected in all the tested sera. The results indicate that YF vaccine induces YFV-specific antibodies in all the tested volunteers and that it also induces HI antibodies cross-reactive for JEV and DV. The YF vaccine has a strong immunogenicity because it is a live vaccine, and induces antibody against YFV predominantly. The international certificate of yellow fever vaccination becomes valid 10 days after vaccination. On day 14th after vaccination, we detected neutralizing antibodies against YFV from all tested volunteers, however, only 6 of 19 volunteers had detectable neutralizing antibody on the 10th day after vaccination. Therefore, the vaccine may not be perfectly effective on day 10th after the vaccination.