Objective : From an analysis of a community outbreak of measles in Youngju, Gyeongbuk, in March 2000, the authors evaluated the effectiveness of the measles vaccination and its determinants to Provide an epidemiologic basis for the establishment of a vaccination policy. Methods : Information was collected regarding the vaccinations from the health records of four primary schools and through a questionnaire surrey of the parents of students in two middle and two high schools (N=4638). Measles cases were surveyed from the patient list of each school and from case reports in the public health center. The attack rate and vaccine effectiveness of measles was evaluated by school; grade; frequency, region, and institution of vaccination. Results : The attack rate cf measles, 6.3% among the total subjects, was higher in middle school students (15.8%) than in primary (2.0%, p<0.05) and high school students (8.9%, p<0.05). The attack rate of the unvaccinated group was 4.6-fold higher than the vaccinated group in primary schools (p<0.05). Vaccine effectiveness was 83.2% among lower graders of primary schools, 34.0% among higher graders of primary schools,26.1% in middle schools, and -7.0% in high schools. In multiple logistic regression analysis, grade in school (odds ratio, 0.77; 95% CI=0.67-0.87) and frequency of vaccination (odds ratio, 0.57: 95% CI=0.37-0.58) were significant predictors of the outcome. Conclusions : We concluded that the explosive outbreak of measles in this area resulted from both inadequate vaccination coverage and secondary failure of vaccination. There was no evidence of any effect of the cold chain system on the vaccine failure.