The prevalence of antibodies to measles, mumps, and rubella in US military recruits is of importance to public health leaders. We performed ELISA testing using a commercially available product on samples from 537 recruits obtained in 1998, of which 437 were positive (81%). We then performed a validation study in a subsample of the population using plaque reduction neutralization (PRN) to assess misclassification error. This resulted in a corrected estimate of the prevalence of immunity to measles of 96% (95% CI: 92-100%). The military vaccinates a percentage of recruits who are likely to be immune if more sensitive testing, such as PRN, was used.