The prevalence of measles antibodies was investigated by an enzyme immunoassay (EIA) in students aged 14 every year since 1996 in a Swiss municipality. This region has wide measles vaccine coverage (first dose > or = 95%, second dose > or = 65%) without any reported measles outbreaks since 20 years. In 2003 and 2004, in contrast to previous years, surprisingly many negative results (33% and 54%, respectively) were observed.