The aim of this study is to prospectively examine electromyographic (EMG) responses in patients diagnosed with acute stress disorder (ASD) after experiencing a traffic accident or violent attack, within one month after the traumatic event and six months later. Half of the participants met criteria for posttraumatic stress disorder (PTSD) after six months. Psychophysiological parameters can provide a better clarification between ASD and PTSD patients. Heightened startle magnitude in the immediate aftermath of trauma may be a good predictor of PTSD; moreover, a lack of startle habituation appears to be a more stable marker of PTSD, which persists for six months after trauma exposure.