Influenza B virus reinfection in Japanese children was studied epidemiologically during 1979-91 and virologically during 1985-91. During this investigation, there were four epidemics caused by influenza B viruses, each of which accompanied antigenic drift. Between the epidemics in 1987/88 and 1989/90, the viruses changed drastically, both genetically and antigenically. The minimum rate of reinfection with influenza B virus during the whole period was 3-25% depending on the influenza seasons. The antigens of primary and reinfection strains of influenza B virus isolated from 18 children during 1985-90, which covered three epidemic periods, were studied by haemagglutination inhibition tests. The results showed that the viruses isolated in the 1984/85 and 1987/88 influenza seasons, which belonged to the same lineage, were antigenically close, and reinfection occurred with these viruses. The results of amino-acid analysis of the HA1 polypeptide of these viruses corresponded with those of antigenic analysis. There were no specific amino-acid changes shared by the primary infection and reinfection influenza B viruses; the patients were infected with the viruses epidemic at that time.