OBJECTIVE: The authors investigated acute stress disorder and later posttraumatic stress disorder (PTSD) in children and adolescents who had been involved in assaults or motor vehicle accidents. METHOD: They interviewed 93 patients 10–16 years old who were seen in an emergency department for having been assaulted or involved in a motor vehicle accident within 4 weeks after the assault or accident to assess acute stress disorder. At 6 months, they reinterviewed 64 (68.8%) of the patients to assess PTSD. RESULTS: At initial interview, 18 (19.4%) of the 93 patients had acute stress disorder and 23 (24.7%) met all acute stress disorder criteria except dissociation. At 6 months, eight of the 64 patients (12.5%) had PTSD. Acute stress disorder and PTSD did not differ in prevalence between patients who had been assaulted and those who had been in accidents. Sensitivity and specificity statistics and regression modeling revealed that the diagnosis of acute stress disorder was a good predictor of later PTSD but that dissociation did not play a significant role. CONCLUSIONS: Acute stress disorder has merit as a predictor of later PTSD in children and adolescents, but dissociation has questionable utility.