PURPOSE: Despite of the high MMR vaccine coverage, the mumps outbreaks is continued in most developed countries including South Korea. To evaluate the effectiveness of MMR vaccine, we carried out mumps outbreak investigation in one kindergarten. METHODS: In Dec. 2006, a mumps outbreak occurred in a kindergarten. Retrospective study was conducted among 212 respondents (205 children and 7 teachers) in 230 study populations (study participation rate: 92%). To define mumps cases, a questionnaire survey, telephone survey, and mumps antibody test with serum were conducted. MMR vaccination status was verified in 198 children using children's vaccination records, National Immunization Registration System and/or medical records of private clinics or hospitals. RESULTS: Over 90% children had received one dose of MMR vaccination. However, 2nd dose of MMR vaccination rate was 45.1%. Attack rates of mumps were 22.9%(22/96) for one dose MMR vaccinees and 3.3%(3/92) for two doses vaccinees. MMR vaccine effectiveness of two doses over one dose was 86%. Mumps attack rate in one dose MMR vaccinees increased by the duration after MMR vaccination. Among all mumps cases, asymptomatic mumps infection identified by IgM positive or IgG over 10,000mIU/ml was 58% (26/ 45). CONCLUSIONS: MMR one dose coverage rate was high(92.1%), but the second dose vaccination rate was very low (45.1%). Mumps attack rate was increased by the duration after the MMR vaccination, and the maximum effectiveness of one dose MMR vaccination was low (77.1%). Therefore, to prevent mumps outbreaks in the kindergarten, second MMR vaccination should be scheduled in 4 years old age, and the coverage rate should be increased over 90%. Further studies for the effectiveness of MMR two doses after 5 and more years are needed.