This study examined the association between cognitive flexibility and variables that may be associated with treatment outcome for 579 veterans seeking Veterans Health Administration treatment for posttraumatic stress disorder (PTSD) secondary to combat or sexual trauma. Factors associated with severity of PTSD (level of PTSD symptoms and guilt cognitions) and with PTSD prognosis (posttraumatic growth and optimistic expectations for the future) were examined. Regression analyses revealed that cognitive flexibility was associated with lower levels of PTSD symptoms and fewer guilt cognitions. Cognitive flexibility was positively associated with posttraumatic growth and optimistic expectations for the future, even when controlling for PTSD severity. These results suggest that interventions designed to increase cognitive flexibility in veterans may be a worthwhile adjunct to treatment for PTSD as we continue efforts to improve treatment outcomes.