Inmates represent a vulnerable population with increased rates of trauma and posttraumatic stress disorder (PTSD). However, little is known about the rates of trauma and PTSD among male inmates with acute psychiatric illness. This prospective, randomized study was conducted to assess the current rates of trauma and PTSD in this population. The sample consisted of 48 patients admitted to a hospital jail psychiatry service in New York City. Subjects were administered the Life Stressor Checklist-Revised and the Structured Clinical Interview for DSM-IV-TR Disorders, PTSD Module (SCID-I). The rate of PTSD diagnosis via SCID-I was 46.2% as compared to 2.1% diagnosis via clinical interview. All participants reported a history of at least one stressful and/or traumatic event, and many of these events occurred during incarceration. These results demonstrate that a great deal of trauma and PTSD goes unrecognized and untreated in this population, indicating the need for more effective treatment interventions.