Serum antibodies against measles were measured in the Dutch general population and in municipalities with low vaccine coverage, where religious groups that refuse vaccination are clustered sociogeographically. The results suggest that wild measles virus may still circulate in municipalities with low vaccine coverage; the circulation in the general population seems to have decreased significantly right after the introduction of mass vaccination. The overall prevalence in the general population was high (95.7%, 95% confidence limits 95.3-96.2%); the seroprevalence in the age groups offered two vaccinations (91.7%, 95% confidence limits 89.4-94.0%) was lower than the level believed to be necessary for the elimination of measles. Protective levels of maternal antibodies in newborns have waned several months before the first vaccination is scheduled.