There are few available data on how to accurately screen for and assess posttraumatic stress disorder (PTSD) among severely mentally ill adults, a group with high rates of unrecognized trauma and PTSD symptoms. We examined the diagnostic utility of a widely used screening instrument, the PTSD Checklist (PCL), for diagnosing PTSD among 44 traumatized, adult, public-sector mental health patients recruited through a community mental health program. Participants completed the PCL and the Clinician-Administered PTSD Scale (CAPS), which is considered the “gold standard” for determining PTSD diagnoses. Data provide preliminary support for the use of the PCL as a screening instrument in public psychiatric settings, indicating that the optimal cut-point for adults with severe mental illness is about 54 (with slightly higher or lower recommended cut-points depending on the clinical context and purpose of the PCL). Such data are critical to ensuring that public-sector mental health patients with trauma-related difficulties are identified and referred for appropriate services. Depression and Anxiety 24:124–129, 2007. Published 2006 Wiley-Liss, Inc.