The investigators propose to conduct a randomized double-blind, parallel-group, placebo-controlled trial of mifepristone in veterans with military-related posttraumatic stress disorder (PTSD). This study will examine the clinical, neuropsychological, and neuroendocrine effects of short-term treatment of mifepristone (600 mg/day for one week) to determine if this treatment is efficacious in improving PTSD symptoms, cognitive functioning, or other related clinical measures. Additionally, the investigators will observe whether baseline neuroendocrine activity, or other clinical or neuropsychological factors predict the response to mifepristone, and whether mifepristone-induced changes in neuroendocrine activity are associated with treatment outcome.