The introduction of a measles vaccination programme in Norway in 1969 using one dose of vaccine, and since 1983 two doses, was followed by a substantial decrease in the incidence of the disease. Since 1992, the annual incidence has been less than 20 cases. Small clusters and outbreaks have occasionally been observed among military personnel and unvaccinated children. This paper describes a seroepidemiological investigation of the level of immunity among 1,188 military conscripts, aged 18-28 years (mean 20.7) compared with 695 healthy 40-year-olds. The conscripts had been offered measles vaccine in infancy, in some cases also at 12-13 years of age, but they had also been exposed to wild measles virus, since the virus continued to circulate many years after the vaccination had started. The measles immunity in this group is considered to indicate the immunity level among the first 5 cohorts offered measles vaccine in Norway. The 40-year-olds had grown up in a community with no measles vaccination. Their level of immunity gives an indication of the level finally obtained when there are no vaccinations, and thus of the level that would induce herd immunity against measles in the Norwegian population. The aims of the vaccination programme must be to obtain a corresponding immunity. The results of the investigation show that the percentages with measles antibodies in the respective groups were 92.3 and 98.1. The observation of measles outbreaks among young Norwegian conscripts, as well as reports from several countries on outbreaks in university and college settings with levels of seropositivity of even more than 95%, indicate that the seropositivity in the 20-year-old group may be too low to afford protection, especially when this group is living under close conditions. Consideration should be given to the need for an intensification of the existing vaccination programme to ensure that the protection level needed for herd immunity is reached.