Objective: Although symptoms of posttraumatic stress disorder (PTSD) are common following exposure to a traumatic event, most people who experience trauma do not develop PTSD. Thus, the identification of risk factors that may interact with trauma exposure to confer vulnerability for the development of PTSD may highlight important targets for prevention and treatment. Recent research suggests that sleep disturbance amplifies the effect of maladaptive emotional processes on PTSD symptom severity. However, no study to date has examined the impact of sleep disturbance, such as insomnia symptoms, on the relationship between trauma exposure severity and specific PTSD symptom clusters. Method: The present study examined insomnia symptoms as a potential moderator of the relationship between trauma exposure severity and specific PTSD symptom clusters among combat exposed veterans (N = 72). Results: Results revealed large associations between insomnia symptoms and PTSD symptom clusters, small to moderate associations between combat trauma severity and PTSD symptom clusters, and a significant interaction between insomnia symptoms and combat exposure to predict reexperiencing, but not avoidance or arousal, symptoms of PTSD. Conclusion: These findings suggest that poor sleep may be one risk context in which trauma exposure results in the development of reexperiencing symptoms of PTSD. The implications of these findings for the development and maintenance of reexperiencing symptoms of PTSD are discussed.