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Previous measles elimination goals have failed in the United States despite high coverage of schoolchildren with a single dose of measles vaccine. Since 1989 advisory groups have recommended that schoolchildren receive a second dose of measles vaccine as part of a revised strategy to eliminate measles from the US. States have responded by phasing in requirements for a second dose of measles vaccine at school entry for various age groups at primary school entrance, secondary school entrance or both. The purpose of this analysis was to evaluate the effectiveness of the requirements for a second dose of measles vaccine and to determine whether a primary or secondary school entrance requirement was more effective in lowering measles incidence.Using national surveillance data we examined the influence of state requirements for the second dose of measles vaccine on measles incidence from 1993 through 1995.Overall measles incidence was lower in states that had a requirement for a second dose of measles vaccine at either primary school entrance [relative risk (RR) = 0.35; 95% confidence interval, 0.25 to 0.49] or secondary school entrance (RR = 0.38; 95% confidence interval 0.29 to 0.50), compared with states without a second dose requirement. Incidence was lowest in states that required a second dose of measles vaccine at both primary and secondary school entrance (RR = 0.22; 95% confidence interval, 0.13 to 0.37).Our findings demonstrate that a requirement for a second dose of measles vaccine at either primary or secondary school entrance is effective in lowering measles incidence, with a greater reduction occurring in states where the second dose is required for both age groups.