Immunization to eliminate measles is recommended at 15 months of age with the option of giving vaccine at 6 to 9 months of age during measles outbreaks in Korea. Because of the recent resurgence of measles and concern about the possibility of reduced vaccine efficacy caused by genomic differences between vaccine virus and contemporary wild measles viruses, we conducted a measles vaccine efficacy study involving children with household exposure ages 1 to 5 years during measles outbreak that had occurred 1993 in Seoul and Seong-nam city, with the demographic analysis of patients brought to the hospitals. A total of 380 patients (M:F = 216:164) were included in this study. Two hundred nine cases (55.0%) occurred in children less than 5 years of age, and 167 (43.9%) were younger than 16 months of age. The recorded age-specific incidence rates showed bimodal patterns, i.e. highest peak in those below 16 months of age and second peak in those ages 6 to 9 years of age. Only 9.6% (16 of 167) of the measles cases less than 16 months, 59.5% (25 of 42) of those 16 months to 4 years and 91.8% (157 of 171) of the cases in school age children have been vaccinated. Attack rates among the 122 vaccinated siblings and 12 unvaccinated siblings ages 1 to 5 years who contacted measles were 5.7 and 75%, respectively, and the clinical vaccine efficacy was 92.4% (95% confidence interval, 83.6, 96.4). The high vaccine efficacy in household exposures suggests that measles outbreaks in Korea are not caused by reduced vaccine efficacy.