On June 21, 1999, a cluster of five cases of measles was reported among the 390 students attending a religion-affiliated elementary school in The Netherlands. Persons belonging to this religious denomination routinely do not accept vaccination. Municipal health services (MHSs) investigated and found 160 suspected measles cases among children attending the school. By February 4, 2000, 2961 measles cases, including three measles-related deaths, had been reported by 35 MHSs to the national registry. This report summarizes the investigation of the measles outbreak in The Netherlands, which indicated that measles can be a severe disease among unvaccinated populations in The Netherlands.