Post-traumatic stress disorder (PTSD) is the name of what was previously identified as traumatic neurosis. This disorder remains controversial because of its psychopathology (trauma/stress), its nature (directly reported when a traumatic event occurs) or its criticism (society medicalization). Despite this, epidemiological studies has demonstrated the sheer number of people suffering from PTSD, but also the medico social cost associated with its chronic nature, the extent of the comorbidities and its broader impact. The occurrence of PTSD is currently well known, as has been supported by several research studies in the general population and more specifics populations such as veterans, women, children.... However, research still remains limited among patients suffering from mental illness, who has a higher than normal risk of being exposed to a traumatic experience. We have studied the prevalence of PTSD but also its characteristics (pretraumatic, peritraumatic, posttraumatic risk factor, comorbidity, suicidality) and its effects (quality of life, functional disability, utilization of health care services) on an representative sample of the target population utilized in this study (n=108). We adapted our methodology to a specific location (emergency department) and to the focus population (patients suffering from mental illness). Our study protocol involved a common evaluation of the sample as a whole, using common statistical tools (questionnaire to collect socio demographic data, WHOQOL BREF for quality of life, functional disability Sheehan scale, utilization of health care services) and an additional evaluation aimed at individuals with a history of trauma, based on specifics tools validated in the psycho trauma domain (CAPS-1, PDI, PDEQ, MINI.) On completion of these evaluations four groups were created which were then analyzed and compared; Individuals who have been exposed to traumatic events vs. individuals who have never been exposed to traumatic events vs. individuals with PTSD vs. individuals without PTSD. The main results of our study demonstrates a greater exposure rate to traumatic events (65,7%) and a greater prevalence of PTSD (13,8%) in the sample compared to the general population. The development of PTSD was statistically significant related to the number of experienced events and to the characteristics of the peritraumatic reaction (distress score and psychic dissociation). It was also linked to a large comorbidity (in particular mood disorders and other anxiety related disorders) and a statistically significantly increase of the suicidality of the individuals with PTSD in comparison with the others was found. The quality of life deteriorated while functional disability increased for the individuals with PTSD in comparison with the others. However the importance of the found comorbidities didn't allow to directly incriminate the PTSD role. Finally, the utilization of health care services was larger for the individuals with PTSD but not statistically significantly in comparison with the others. Conclusion: There is clearly an under diagnostic of PTSD among the patient population who are both suffering from mental illness and are consulting the emergency departments for a psychiatric reason. The characteristics of this trouble and its broader impact described in our study warrant a search of the history of trauma in the same way as the psychiatric and somatic history. Moreover in the case of history of trauma and despite the comorbidities importance, this trouble should be diagnosed (thanks to an active approach of the clinician) and the recommended therapeutic care should be favored and in particular by allowing a better orientation for the urgency department.