The purpose of this study was to evaluate the effects of a Group Psychological Debriefing (GPD) on acute stress reactions and perceptions following a traumatic situation. A group of military personnel (N = 9) and a group of voluntary civilian firefighters (N = 9) were exposed to severe stress during rescue work in a tunnel following a fatal traffic accident. Both groups participated in an operational debriefing and received brief stress management counseling after the incident. In addition the military personnel also participated in a structured 2.5 hour GPD after the accident. Two weeks later both groups completed the Coping Style Questionnaire (CSQ-30), the Impact of Event Scale (IES), the Post Traumatic Symptom Scale (PTSS-10), the General Health Questionnaire (GHQ-30) and questions about stress and coping after the incident. Lower frequency of symptoms were found in the GPD compared to the non-debriefed group measured by the PTSS-10. Single item analyzes of the scale revealed lower levels of symptoms related to emotional arousal in the GPD participants. Furthermore, the GPD participants revealed more positive personal outcomes than the non-debriefed group after the accident. The total PTSS-10 score and mental preparedness were the two variables that were best able to classify participants as belonging to the two groups, thus indicating significant clinical differences between the debriefed and non-debriefed group and the potential usefulness of PTSS-10 scale as a brief screening inventory after traumatic events.