This Phase IIa clinical trial seeks to enroll 90 eligible male combat Veterans with chronic PTSD. Participants will be randomly assigned to treatment with 600 mg/day mifepristone or placebo for one week and assessed for clinical outcomes at one and three months follow-up. Eligibility will be based on the inclusion and exclusion criteria which are enumerated below and in the Human Participants section. The investigators plan to include male combat Veterans with chronic PTSD who are not receiving psychotropic medication. The inclusion and exclusion criteria were selected in order to include as representative a sample as possible while also addressing safety concerns. Veterans who are actively suicidal as assessed by the Columbia-Suicide Severity Rating Scale (C-SSRS) or who have attempted suicide within the past two years will be excluded. (If suicidality is identified, the necessary steps will be taken to ensure the appropriate clinical care is provided, and the local suicide prevention policies implemented.) Veterans with adrenal insufficiency will be excluded, as will Veterans with other major medical or neurological illnesses, as they may be at increased risk of developing adverse events. Veterans with renal disease/impairment, hepatic disease/impairment, cardiac illness (e.g. coronary vascular disease, congestive heart failure), or hypokalemia at screening will also be excluded. Since mifepristone use can prolong the corrected QT (QTc) interval in a dose-related manner, Veterans with a prolonged QTc interval, defined as >450 msec, on the ECG at screening will be excluded. To evaluate QTc prolongation post-mifepristone treatment, an ECG will also be performed at both the 3 day follow up visit (visit 1.5) and 1 week follow-up visit (week 1, visit 2).