Relationships among a modifiable situational factor (unit cohesion), prior stressful life events, and posttraumatic stress disorder (PTSD) symptoms were assessed in 1,579 U.S. Army soldiers with no history of contemporary war zone deployment. It was predicted that unit cohesion would attenuate the dose-response relationship between past stressor exposures and PTSD symptoms at relatively moderate levels of exposure. Consistent with this hypothesis, regression analysis revealed that life experiences and unit cohesion strongly and independently predicted PTSD symptoms, and that unit cohesion attenuated the impact of life experiences on PTSD. Some military personnel reported significant predeployment, stress-related symptoms. These symptoms may serve as vulnerabilities that could potentially be activated by subsequent war-zone deployment. Higher predeployment unit cohesion levels appear to ameliorate such symptoms, potentially lessening future vulnerability.