Since the introduction of routine measles, mumps and rubella immunization for children in England and Wales in 1988, the incidence of mumps has declined steadily. We describe an outbreak of mumps in 1996 attacking 34 of a cohort of 98 schoolchildren born in 1982 and 1983. This is the largest outbreak in the UK since the introduction of the vaccine into the childhood immunization schedule. Salivary IgM assay was used as a simple, minimally invasive test to confirm the diagnosis. The occurrence of the outbreak demonstrates that British children who were just too old to receive mumps immunization in 1988 continue to be at risk of this disease as a result of diminished natural exposure. Further cases and outbreaks in this cohort are to be expected. Cohorts born before 1982 appear to be at less risk, presumably because of naturally acquired infection before the introduction of immunization.