Military deployment can create significant relationship strain. Although most couples navigate the challenges of deployment successfully, this period may render some couples more vulnerable to adverse relationship outcomes such as infidelity due to a convergence of factors including geographic separation and reduced emotional and physical intimacy. Despite anecdotal reports of increased rates of infidelity during deployment, empirical findings are lacking. This study used a prospective design to examine the prevalence and risk factors of infidelity across the deployment cycle including a year-long deployment to Iraq. A total of 63 married male Airmen were assessed both pre- and 6–9 months postdeployment. The rate of sexual infidelity prior to deployment (21%) was commensurate with the lifetime rate of sexual involvement outside the marriage in representative community samples of men. Across the deployment period, the prevalence of sexual infidelity was strikingly high (22.6%) compared with annual community estimates (1.5–4%; Allen et al., 2005). Findings demonstrated that service members with a prior history of separation, steps toward divorce, and relationship distress prior to deployment had elevated risk for infidelity over the deployment cycle. Moreover, roughly 75% of Airmen who experienced infidelity over the deployment cycle divorced by 6–9 months postdeployment whereas only 5% of service members without infidelity divorced during this same time period. Considering well-documented adverse impacts of infidelity and divorce, the current findings may assist in identifying military couples at risk for infidelity and informing targeted prevention or early intervention strategies for these couples prior to or immediately following deployment.