In Spain, measles, mumps and rubella vaccination was introduced in 1981, with one dose at the age of 15 months and another at the age of 11 years being administered since 1995. Reported disease incidence was less than one case per 100,000 people for measles and rubella, and 23 cases per 100,000 people for mumps. A seroepidemiological survey was undertaken to estimate the frequency of susceptible individuals by age and environment; and vaccination coverage and efficacy of the vaccines administered. A population-based cross-sectional study was then conducted, covering the population aged 2-39 years, residing in Spain (excluding Catalonia). The sample was stratified by age and rural/ urban environment and informed consent obtained to take blood specimens from subjects attending blood-extraction centres. The final sample totalled 3,932 persons. IgG antibodies were detected by an enzyme-linked immunosorbent assay. Estimated vaccination coverage was 96% for children aged 2-5 years; vaccine efficacies were 96.7% for measles, 97.2% for rubella and 79.3% for mumps. Immunity was the lowest in the 6-9 year age group for measles (90.8%) and in males aged between 15 and 24 years for rubella (86 and 89.8%, respectively). In the case of mumps, this proved the lowest in the 2-5 year age group (76.7%) and in those autonomous regions in which only the Rubini strain had been administered. The incidence of measles has enabled the National Measles Elimination Plan to be implemented by which the elimination of congenital rubella syndrome could now be initiated. A possible explanation for the higher susceptibility observed for mumps might lie in the Rubini strain's low efficacy.