A catch-up campaign targeting children aged 8-16 years using measles-rubella (MR) vaccine was conducted during 2001 in Korea. To evaluate the impact of the campaign and assess mumps immunity, human IgG antibodies were detected using ELISA for measles (5826 samples) and mumps (5890 samples) in a national sample of opportunistically collected sera from a population aged 0-34 years. The measles immunity increased by 5-10% following the catch-up campaign in the targeted age group. Infants lost maternal antibodies rapidly and about 90% of infants were susceptible to measles and mumps at 6-8 months of life. The sero-prevalence of mumps antibody increased slowly with age and stabilized at a lower level when compared with that of measles. Despite an immediate reduction in susceptibility among the targeted age group of the catch-up campaign, continuous efforts to increase immunization coverage are needed to interrupt indigenous measles transmission. Furthermore, our results suggest continuous mumps outbreaks could occur because of the accumulation of susceptible individuals.