Recent mumps outbreaks have evoked concerns of decreasing mumps immunity among adolescents and adults, including US military recruits subject to differing mumps immunization policies. To compare mumps incidence and to assess initial measles, mumps and rubella seropositivity, we conducted a cohort study among recruits from 2000 to 2004. Mumps incidence in the targeted MMR and universal MMR cohorts was 4.1 and 3.5 per 100,000 person-years, respectively, giving an incidence rate ratio of 1.16 (P=0.67). Measles, mumps, and rubella seropositivity was 84.6%, 89.5%, and 93.2%, respectively. Among recruits with measles and rubella immunity, 92.8% were mumps immune. These findings support the policy of targeting MMR immunization based upon measles and rubella serology alone.