Hungary has had a successful measles vaccination programme, achieving over 93% coverage in targeted groups. However, from September 1988 until December 1989, 17,938 measles cases were reported among the civilian population (attack rate [AR] = 169 per 100,000 population) with the majority of cases occurring in vaccinated people. National surveillance data were analysed to determine reasons for the outbreak and risk factors for vaccine failure. People born during 1971 and 1972 had been targeted for vaccination during campaigns in April and September of 1973 and had the highest AR (1332 and 1632 per 100,000, respectively). Epidemiological studies of vaccine efficacy conducted among secondary school students corroborated these findings. Among 754 secondary school students, those vaccinated during the April 1973 campaign were at highest risk compared with those vaccinated at routine health care after 1974 (relative risk = 10.9, 95% confidence interval [Cl]: 2.5-47.9). Among 341 primary school students, one-dose recipients were at higher risk compared with two-dose recipients controlling for age at and time elapsed since vaccination (P = 0.04).