In 1991, in Fukuoka, Japan, a measles outbreak occurred in which we observed 15 cases of measles vaccine failure (MVF). We examined these patients both clinically and serologically. Seven of them, with a response pattern of an early rise in and attainment of a high hemagglutination inhibition (HI) antibody titre, were considered to be secondary vaccine failures (EH group). Eight others showed a normal (relatively late rise and low titre) HI antibody response pattern and were considered to be primary vaccine failures (LL group). In both MVF groups, measles-specific IgM antibody was detected by enzyme immunoassay. The EH group had a milder rash than did the LL group and unvaccinated controls. We believe they had an immunological memory that modified the clinical manifestations of measles. Two cases of encephalitis were observed in the EH group; both patients recovered without sequelae. These data suggest that the mere presence or absence of IgM antibody is not sufficient to differentiate primary from secondary MVFs. A two-dose measles vaccination scheme should be recommended to secure a booster effect, because immunity is waning in the population in which the measles vaccination rate is not high enough and in which natural measles still exists.