Posttraumatic stress disorder (PTSD) is defined by DSM-IV-TR (American Psychiatric Association 2000) as a mental disorder that follows exposure to a traumatic event in which the person experienced, witnessed, or was confronted with actual or threatened death or serious injury, or threat to the physical integrity of self or others, provoking terror, horror, or helplessness at the time of exposure and resulting in symptoms of reexperiencing, avoidance and numbing, and hyperarousal lasting more than a month and causing social or occupational dysfunction. Acute stress disorder (ASD) was added to the diagnostic nomenclature in DSM-IV (American Psychiatric Association 1994) to describe a syndrome of severe dissociative, intrusion, avoidance, and hyperarousal symptoms associated with occupational and social dysfunction in the first 4 weeks after a traumatic stressor. It was included in the nosology because of evidence that a substantial minority of trauma victims endure these symptoms and that these symptoms are associated with an elevated risk of progressing to PTSD (Koopman et al. 1994). By definition, PTSD cannot be diagnosed until 1 month after the occurrence of a traumatic stressor, and the adjustment disorders were not thought to reflect the severity of either the stressors or the reaction to them. The diagnosis of ASD requires the presence of at least three dissociative symptoms, in addition to one each of the intrusion, avoidance, and hyperarousal symptoms seen in PTSD.