After vaccination of Dutch children against mumps started in 1987 through the National Immunisation Programme the incidence of mumps decreased greatly. However, outbreaks of mumps have been occurring since the end of 2009, especially among students. We describe a vaccinated 20-year-old woman with uncomplicated mumps, a vaccinated 20-year-old student with mumps orchitis, and an unvaccinated 14-year-old boy who developed one-sided permanent deafness as a complication of mumps. Mumps outbreaks are likely caused by factors including incomplete protection following vaccination, waning of immunity, and intensive crowding. Mumps currently affects mainly people who were vaccinated more than 10 years ago. Outbreak response concentrates on surveillance, outbreak investigations, disease awareness, and provision of catch up vaccination to unvaccinated or incompletely vaccinated students. Adequate surveillance, important for building evidence for possible changes in mumps vaccination recommendations, depends on recognizing mumps and its complications.