OBJECTIVES: Although there were several campaigns to eradicate measles from US by immunization, the outbreak of measles in 2005 was one of the major indicators of re-emergence of measles. What is the impact of 2005 measles outbreak on immunization rate (against measles) in the following year (2006)? METHODS: A retrospective cohort study was conducted using National Immunization Survey (NIS) 2004 and NIS 2006 datasets. The outcome variable was up-to-date immunization record against measles. The treatment variable was the outbreak of measles in 2005. Year 2004 was considered for pre-outbreak period and year 2006 was considered for post-outbreak period. Age, gender, race/ethnicity, Hispanic origin and first born status of child; number of people in household; age, marital status and education of mother were controlled to obtain unconfounded causal relation. Therefore, assumption of ignorability was met for this study. The stable unit treatment value assumption was also met because of the nature of the study. Propensity score matching method was used to make two groups comparable. Confound adjusted logistic regression (using weighted matched controls) was conducted to estimate the change in measles immunization rate in 2006 from 2004. RESULTS: A total of 60,867 data was collected during 2004 and 2006 NIS. Weighted population totals 11,871,692 observations, representing an average of the US population of 19-35 month old children over the survey years. Confounders have shown good percent reduction in bias after propensity score matching. Odds ratio was less than 1 and also significant (OR=0.560, p=0.048) only for the treatment variable. CONCLUSIONS: Immunization rate against measles has been decreased after the largest measles outbreak, and this is a concerned matter in the perspective of public health. Community based and nation-wide intervention for immunization is still necessary.