This research assessed the lifetime prevalence of traumatic events and current posttraumatic stress disorder (PTSD) in 275 patients with severe mental illness (e.g., schizophrenia and bipolar disorder) receiving public mental health services in Concord and Manchester, New Hampshire, and Baltimore, Maryland. Lifetime exposure to traumatic events was high, with 98% of the sample reporting exposure to at least 1 traumatic event. The rate of PTSD in our sample was 43%, but only 3 of 119 patients with PTSD (2%) had this diagnosis in their charts. PTSD was predicted most strongly by the number of different types of trauma, followed by childhood sexual abuse. The findings suggest that PTSD is a common comorbid disorder in severe mental illness that is frequently overlooked in mental health settings.