The Czech Republic has had a two-dose measles, mumps and rubella (MMR) vaccination programme since 1987. The last outbreak of mumps was reported in 2002, but an increase in the number of mumps cases was observed in 2005, starting in October that year. We analysed routinely collected surveillance data from 1 January 2005 to 30 June 2006 to show the magnitude of the increase and describe the most affected groups in order to better target prevention and control strategies. In the 18-month period examined, 5,998 cases of mumps were notified, with a peak incidence in May 2006. No deaths were recorded, but 21% of cases were hospitalised. Incidence was lowest in the Plzen region (1.9/100,000) and highest in Zlin (118.6/100,000). There were more male (61.8%) than female cases. The age of the cases ranged from 0 to 80 years. The highest incidence rate was observed in the age group of 15 to 19 years, in which 87% of cases had received two doses of mumps vaccine. The average age of unvaccinated cases was 22.9 years, while for cases vaccinated with two doses it was 14.5 years. Although vaccine effectiveness could not be calculated from the data available, possible reasons for highly-vaccinated cases occurring are discussed.