A two-dose vaccination programme, using a combined measles, mumps and rubella vaccine (MMR) and administration at the ages of 18 months and 12 years, was introduced into Sweden in 1982. Since the combined MMR vaccine was introduced, a yearly evaluation of the immunity patterns and seroconversion rates in 12-year-old children has been carried out. This study includes three study groups. All groups consisted of pre- and postvaccination samples from 12-year-old children taken in connection with the MMR immunization. There were 332 paired samples from children who 8-10 years previously had received live measles vaccine. Prevaccination sera from these children represent late postvaccination titres. The postvaccination sera of 49 children seronegative to measles before vaccination, obtained 2 months after vaccination, represent early postvaccination immunity. Ninety-five children who had not been vaccinated earlier and who were seropositive to measles represent naturally acquired immunity. All samples were studied by an enzyme-linked immunosorbent assay (ELISA). Sera from children with late postvaccination immunity had significantly lower titres than children in group 2, who represented early postvaccination sera (p < 0.001). Children with naturally acquired immunity had significantly higher titres than children with vaccine-induced immunity titres (p < 0.001). After revaccination of the previously vaccinated children, a significant booster rise was seen (p < 0.001). After revaccination of children with naturally acquired immunity, no significant booster effect was observed. Sixty-five children of the 332 (20%) previously vaccinated children had no or borderline measles titres prior to the booster. The study suggests that the vaccine-induced measles antibodies decline with time and may fall under the protective level.(ABSTRACT TRUNCATED AT 250 WORDS)