Certain individuals, such as police officers, are exposed to traumatic events as part of their occupational roles. In an effort to prevent psychological illnesses, notably the post-traumatic stress disorder, from arising out of work-related traumatic incidents, psychological interventions have been developed such as Critical Incident Stress Debriefing (Mitchell, 1983; Mitchell & Everly, 1996). The present study tests the hypothesis that debriefing reduces the psychological morbidity caused by work-related incidents. Because debriefing techniques were not designed for application on a ‘one-off’ basis (Robinson & Mitchell, 1993), the procedure studied here consisted of three successive debriefing sessions (at 24 hours, 1 month and 3 months post-trauma), which included traumatic stress education. In a sample of 243 traumatized police officers, a subgroup of debriefed officers (N = 86) was compared with non-debriefed internal (N = 82) and external (N = 75) control groups. No differences in psychological morbidity were found between the groups at pre-test, at 24 hours or at 6 months post-trauma. One week post-trauma, debriefed subjects exhibited significantly more post-traumatic stress disorder symptomatology than non-debriefed subjects. High levels of satisfaction with debriefing were not reflected in positive outcomes. The findings are translated into recommendations for the future use of debriefing in police practice.