Harbertson J, et al. – In this first-in-kind study, researchers assessed the prevalence of condom use among ship-assigned US military personnel across an overseas deployment cycle, and identified factors associated with condom non-use. They obtained longitudinal survey data from ship-assigned US Navy/Marine Corps personnel on 11 ships before (T1), during (T2) and after (T3) an overseas deployment. The survey was anonymous, self-completed, and included information related to demographics, condom use at last sex, STI diagnosis, alcohol misuse and drug use with sex. The analysis included 1,900 (T1), 549 (T2) and 1,168 (T3) personnel who reported age, sex, and condom use/non-use at last sex. The proportion reporting condom use was significantly higher during T2 (53%) compared with T1 (27%) or T3 (28%)—with STI prevalences of 1% (T1), 7% (T2), or 2% (T3), and fewer (29%) sexually active individuals at T2. In adjusted models, condom non-use was linked to hazardous alcohol use or drug use to enhance sex, but transactional sex was negatively associated. Overall, condom use and STI prevalence were greatest during deployment. In an effort to reduce STI transmission, these data could potentially be utilized to promote targeted interventions and spread to similarly aged cohorts traveling outside the United States.