This study examined the impact of killing on posttraumatic stress symptomatology (PTSS), depression, and alcohol use among 317 U.S. Gulf War veterans. Participants were obtained via a national registry of Gulf War veterans and were mailed a survey assessing deployment experiences and postdeployment mental health. Overall, 11% of veterans reported killing during their deployment. Those who reported killing were more likely to be younger and male than those who did not kill. After controlling for perceived danger, exposure to death and dying, and witnessing killing of fellow soldiers, killing was a significant predictor of PTSS, frequency and quantity of alcohol use, and problem alcohol use. Military personnel returning from modern deployments are at risk of adverse mental health symptoms related to killing in war. Postdeployment mental health assessment and treatment should address reactions to killing in order to optimize readjustment. © 2010 American Psychological Association.