In Belgium, children are immunized against measles-mumps-rubella (MMR) in a two-dose schedule at the age of 15 months and 11 years. Despite these recommendations, epidemics of mumps still occur. During an outbreak of mumps in Bruges (Belgium), 105 cases were registered in seven schools (age group 3-12 years). Lower than optimal vaccination coverage, inadequate vaccination schedule and a combination of primary and/or secondary vaccine failure are considered as possible reasons for the outbreak as described in the article. The role of secondary vaccine failure is highlighted.