Posttraumatic stress disorder (PTSD) is common among treatment-seeking substance abusers. Despite the high prevalence of these co-occurring conditions, few PTSD screening tools have been evaluated for their utility in identifying PTSD in substance use disorder (SUD) populations. The present study evaluated the psychometric properties of the Impact of Event Scale-Revised (IES-R) in a sample of 124 substance dependent individuals. All participants had a history of a DSM-IV Criterion A traumatic event, and 71 individuals met diagnostic criteria for PTSD. Participants with comorbid PTSD reported significantly more symptoms of anxiety, depression, and PTSD compared to substance dependent individuals without PTSD. Acceptable internal consistency and convergent validity of the IES-R were established among a substance dependent sample. Examination of diagnostic effectiveness suggested a cutoff value of 22 as optimal for a substance using population, resulting in adequate classification accuracy, sensitivity, and specificity.