Abstract Few prospective studies on pre-trauma predictors for subsequent development of posttraumatic stress disorder (PTSD) have been conducted. In this study we prospectively investigated whether pre-deployment personality and the cortisol awakening response (CAR) predicted development of PTSD symptoms in response to military deployment. Furthermore, we hypothesized that potential effects of age, childhood trauma and previous deployment on development of PTSD symptoms were mediated via pre-deployment personality, CAR and PTSD symptoms. Path analysis was performed on data from 470 male soldiers collected before and six months after a 4-month deployment to Afghanistan. Before deployment, personality was assessed with the short-form Temperament-Character Inventory and the Cook-Medley Hostility scale. In addition, pre-deployment saliva sampling for assessment of the CAR was performed immediately after awakening and 15, 30 and 60min thereafter. Pre-deployment high hostility and low self-directedness represented intrinsic vulnerabilities for development of PTSD symptoms after deployment. The CAR assessed before deployment did not predict PTSD symptoms after deployment. Pre-deployment low-to-moderate PTSD symptoms were associated with PTSD symptoms after deployment. As hypothesized, the effects of age and childhood trauma on PTSD symptoms after deployment were mediated via personality and pre-deployment PTSD symptoms. However, the number of previous deployments was not related to development of PTSD symptoms. The total model explained 24% of variance in PTSD symptoms after military deployment.