In 1996 the effects on the immunity profile of a Swiss population exposed to MMR vaccination, which has been recommended since 1985, were evaluated with an age-stratified seroprevalence study for measles, mumps and rubella (MMR). At the age of 1.5-2.5 years, seroprevalence attained 76% for measles and rubella, which is respectively 17% and 24% above the values observed in 1992. The seroprevalence for mumps attained only 55% at the same age, which could reflect the poor immunogenicity of this component of the MMR vaccine. The seroprevalence for measles IgG showed a slow but steady increase from vaccination age to adulthood, attaining nearly 100%. The concentrations of measles IgG were about 700 IU/l into adolescence and rose to a plateau at about 1500 IU/l during young adulthood. These observations are compatible with low endemic activity of measles in the last 20 years and a predominance of vaccine-induced immunity up to about 20 years of age. This corresponds to the time period when measles vaccines--single or as MMR--have been in use. In 1992, at the peak of epidemic activity, seroprevalence for mumps rose substantially faster than in 1996. In addition, the rapid increase in quantitative values during preschool age mirrors the ongoing wild virus circulation with minimal vaccine effect. In the vaccine cohort (2-12 years of age) the seroprevalence of rubella IgG reached 70-80%. That there is no rise in the curve during school age shows that the recommended catch-up vaccinations before or during school age have been neglected. The median concentrations of rubella IgG were about 65 IU/ml at vaccination, declined to 40-50 IU/ml during preschool age, and rose again during school age, suggesting wild virus circulation. These data show that the MMR vaccine cover in Swiss children is insufficient to interrupt virus circulation, and administration of a second dose of MMR for catch-up immunisation has been omitted. The poor efficacy of the mumps component of the MMR vaccine that has mainly been used in Switzerland is also evident. The average age at infection is therefore expected to rise, thus involving a risk of increasing age-dependent complications. Efforts to implement the MMR vaccination program in Switzerland should be improved.