Objective: Interview data collected from 275 clients with severe mental illnesses are used to test the construct and criterion validity of the Posttraumatic Stress Disorder Symptom Scale (PSS). Method: First, exploratory and confirmatory factor analyses are used to test whether the scale reflects the posttraumatic stress disorder (PTSD) symptom criteria of the "Diagnostic and Statistical Manual of Mental Disorders", 4th ed., text rev. ("DSM-IV-TR"). Second, structural equation modeling (SEM) is used to test concurrent validity. Results: A 2-factor PSS reveals an excellent fit to the data, and SEM shows good concurrent validity with key study variables. Conclusion: The PSS reveals a 2-factor structure (Re-experiencing and Avoidance) in this particular sample, and stress from high-risk behaviors and drinking to cope with negative emotions mediate the relationship between subjective distress from trauma and PTSD symptoms. (Contains 2 tables and 1 figure.)