A mumps outbreak occurred in a group of vaccinated children aged 3-4 years in San Sebastián (Gipuzkoa, Basque Country, Spain) in 2000 during the same period as a revaccination campaign against measles-mumps-rubella (MMR) was performed. The clinical cases were confirmed by viral culture, detection of viral RNA and/or specific IgM. Eighty-eight percent of the children had been vaccinated with the Rubini strain and the remainder with the Jeryl-Lynn strain. The attack rate was 47.9% (35 cases in 73 school-attending children of this age). The outbreak was caused by an H genotype strain of mumps virus which was circulating at the same time as a D genotype strain that caused sporadic cases. By sequencing the small hydrophobic (SH) gene, the strains of the clinical cases were identified as wild-type mumps virus with heterologous genotypes in comparison to the vaccine strains used in our area.