OBJECTIVE: To assess the ability of acute stress disorder to predict posttraumatic stress disorder (PTSD), the relationship between acute stress disorder and PTSD over the 2 years following mild traumatic brain injury was determined. METHOD: Survivors of motor vehicle accidents who sustained mild traumatic brain injuries were assessed for acute stress disorder within 1 month of the trauma (N=79) and for PTSD at 6 months (N=63) and 2 years (N=50) posttrauma. RESULTS: Acute stress disorder was diagnosed in 14% of the patients. Among the patients who participated in all three assessments, 80% of the subjects who met the criteria for acute stress disorder were diagnosed with PTSD at 2 years. Of the total initial group, 73% of those diagnosed with acute stress disorder had PTSD at 2 years. CONCLUSIONS: This study provides further support for the utility of the acute stress disorder diagnosis as a predictor of PTSD but indicates that the predictive power of the diagnostic criteria can be increased by placing greater emphasis on reexperiencing, avoidance, and arousal symptoms.