Purpose : This study was intended to measure seropositivities and the levels of mumps- and rubella-specific IgG of MMR vaccinees over 17 months of age in Korea. Materials and Methods : From June 1994 to April 1995 we obtained sera from visitors of well baby clinic and patients in Korea University Hospital, who were MMR vaccinees over 17 months of age and had no evidence of immunodeficiency. These 275 study population include 145 males and 130 females. Mumps- and rubella-specific IgG antibody levels were measured by ELISA. Cut-off values for seropositivity were 20 GU(Gamma Unit) in mumps and 0.17 in rubella. Results : 1) As age increased, seropositivities of mumps-specific IgG increased significantly, being 69.0% in 1.5~2 year, 75.0% in 3~4 year, 76.0% in 5~6 year, 90.0% in 7 year, 100% in 8 year, 96.9% in 9 year, 97.4% in 10 year, 97.4% in 11 year, and 96.6% in 12 year of age(p<0.001). 2) As age increased, the levels of mumps-specific IgG antibody(mean${pm}$standard deviation, GU) increased significantly, being $64.9{pm}66.5$ in 1.5-2 year, $117.7{pm}126.4$ in 3~4 year, $152.3{pm}147.1$ in 5~6 year, $194.3{pm}168.2$ in 7 year, $258.1{pm}190.6$ in 8 year, $193.1{pm}130.1$ in 9 year, $225.7{pm}119.6$ in 10 year, $220.7{pm}114.3$ in 11 year, and $222.3{pm}127.1$ in 12 year of age(p<0.001). There was positive correlation between age and mumps-specific antibody level (r=0.3282, p<0.001). 3) As age increased, seropositivities of rubella-specific IgG decreased significantly, being 72.4% in 1.5~2 year, 75% in 3~4 year, 72% in 5~6 year, 60% in 7 year, 44.4% in 8 year, 40.6% in 9 year, 28.2% in 10 year, 23.1% in 11 year, and 17.2% in 12 year of age(p<0.001). 4) As age increased, rubella-specific IgG decreased significantly, being $0.462{pm}0.356$ in 1.5~2year, $0.438{pm}0.306$ in 3~4 year, $0.287{pm}0.179$ in 5~6 year, $0.204{pm}0.139$ in 7 year, $0.189{pm}0.153$ in 8 year, $0.124{pm}0.121$ in 9 year, $0.093{pm}0.114$ in 10 year, $0.104{pm}0.135$ in 11 year, and $0.080{pm}0.001$ in 12 year of age(p<0.001). There was negative correlation between age and rubella-specific IgG titer (r=-0.551, p<0.001). Conclusions : Eventhough seropositivities and the level of mumps-specific IgG increased as age increased, they are not enough to prevent mumps infection in 1.5 to 6 years of age. Seropositivities and the level of rubella-specific IgG decreased as age increased. Appropriate change in vaccine schedule may be needed to decrease the risks of mumps and rubella infection.