The study examined dispositional optimism s role in buffering the effect of warzone stress on mental health symptoms and mental health symptoms on work impairment. A total of 2,439 soldiers from an active-duty brigade combat team were surveyed following a 12-month deployment to Iraq. Posttraumatic stress disorder (PTSD) symptoms, depression symptoms, combat exposure, deployment demands, and work impairment were measured. Soldiers higher in dispositional optimism showed weaker relationships between combat exposure and PTSD symptoms, and between deployment demands and PTSD and depression symptoms. Dispositional optimism also buffered mental health symptom effects on work impairment. Dispositional optimism may protect soldiers from warzone stress and mental health symptoms. Potential mechanisms explaining how dispositional optimism may serve as a protective factor are discussed. © 2011 Wiley Periodicals, Inc. J Clin Psychol 67:1–15, 2011.