Acute stress disorder (ASD) is a precursor of chronic posttraumatic stress disorder (PTSD). Twenty-four participants with ASD following civilian trauma were given 5 sessions of either cognitive-behavioral therapy (CBT) or supportive counseling (SC) within 2 weeks of their trauma. Fewer participants in CBT (8%) than in SC (83%) met criteria for PTSD at posttreatment. There were also fewer cases of PTSD in the CBT condition (17%) than in the SC condition (67%) 6 months posttrauma. There were greater statistically and clinically significant reductions in intrusive, avoidance, and depressive symptomatology among the CBT participants than among the SC participants. This study represents the 1st demonstration of successful treatment of ASD with CBT and its efficacy in preventing chronic PTSD.