PURPOSE: To maintain measles elimination status, we evaluated the seropositivity of measles and mumps according to time interval since the first or second dose of MMR in children aged 4 to 6 years, who are starting communal life. MATERIALS AND METHOD: 2,447 children aged 4 to 6 years were enrolled at 251 public health centers over the period of March to May 2007. Subjects were verified their date of MMR vaccination and then their blood was sampled for serologic test. Measles and mumps IgG antibody was tested by ELISA at Korea CDC. RESULTS: Vaccination coverage was 99.9% in the first dose, 64.9% in the second dose regardless of gender. Seropositivity of measles and mumps was 95.7%, 85.5% in the first dose and 98.7%, 98.1% in the second dose, respectively. The seropositivity of measles was 88.1% in 6-year-olds who did not receive the second dose of MMR. As time since receipt, seropositivity of measles tended to decrease over time and was 93.3% in vaccinees over 48 months after the first dose. CONCLUSION: A first dose MMR at 12-15 months cannot lead to herd immunity. More public information is needed to encourage second dose vaccination before admission to day-care center or kindergarten.