Trauma and posttraumatic stress disorder (PTSD) have high prevalence among individuals with severe mental illness, such as schizophrenia or schizoaffective disorder. This study examined whether trauma and PTSD are under-detected in this population, and whether the cognitive theory of PTSD is applicable to these individuals. Traumatic experiences, PTSD symptoms and negative posttraumatic cognitions were directly measured with questionnaires, and compared to information obtained via chart-review. Results showed clear evidence of under-report of trauma and under-diagnosis of PTSD in patients' charts. Furthermore, negative posttraumatic cognitions were positively related to PTSD symptom severity, supporting the cognitive model of PTSD. These findings underscore the importance of assessing trauma history as well as PTSD in the routine evaluation of patients with schizophrenia or schizoaffective disorder in outpatient clinical settings. Furthermore, the finding of negative posttraumatic cognitions suggests that the cognitive model of PTSD may be applicable to patients with schizophrenia or schizoaffective disorder.