Background. The DSM-IV definition of post-traumatic stress disorder (PTSD) requires that the syndrome cause clinically significant distress or impairment. The impact of the clinical significance criterion on the lifetime prevalence of PTSD among civilian victims of traumatic events has not been evaluated. Method. Data from two community-based samples were examined, the 1996 Detroit Area Survey of Trauma (n=2181) and the Mid-Atlantic Urban Youth Study (n=1698). The World Health Organization Composite International Diagnostic Interview (WHO CIDI) was used to ascertain DSM-IV PTSD. Results. The inclusion of the clinical significance criterion in DSM-IV reduces the conditional probability of PTSD given exposure to trauma by approximately 30%. Cases with clinically significant syndrome showed more pervasive and persistent disturbance and an excess in impaired activity days. Conclusions. The consistency of the findings between the two studies strengthens the evidence on the impact of the clinical significance criterion in the diagnosis of PTSD, and the construct validity of its measurement. There is a need for greater research effort on the definition and measurement of the clinical significance criterion. © 2007 Cambridge University Press.