Military personnel returning from the conflicts in Iraq and Afghanistan commonly experience mental health problems and efforts are underway to determine risk and protective factors associated with postdeployment mental health concerns. This study examined the contribution of trait neuroticism, predeployment life events, combat experience, perceptions of threat, and postdeployment social support on mental health symptoms at 6 months, 12 months, and 24 months postdeployment. Two hundred seventy-one veterans completed self-report measures. Hierarchical regression analyses demonstrated that neuroticism predicted post-traumatic stress and depressive symptoms at all 3 time points; perceived threat predicted post-traumatic stress symptoms at Time 1 and Time 2 and depressive symptoms at Time 2. Social support was a strong negative predictor of post-traumatic stress and depressive symptoms. Alcohol misuse was not significantly predicted by any of the variables. The present study highlights the role of perceived threat and trait neuroticism on postdeployment mental health symptoms and indicates social support is a robust protective factor. Efforts aimed at increasing sustained postdeployment social support may help defend against significant mental health problems among veterans.