Objective: To test the hypothesis that treatment with the GR-antagonist mifepristone is more effective than placebo to reduce PTSD symptom severity in treatment-resistant PTSD. Main trial endpoints: Improvement of PTSD symptoms, as measured with the monthly version of the CAPS-5 (Clinician Administered PTSD scale) in patients with treatment-resistant PTSD, 4 weeks after the start of the intervention. Secondary trial endpoints * PTSD symptom severity as measured with the weekly version of the PCL-5, from baseline till 12 weeks after the start of the intervention (T3). * Long-term PTSD symptom severity as measured with the CAPS-5, at 12 weeks after the start of the intervention (T3). * Loss of diagnosis (score of \<26 and absence of PTSD criteria with CAPS-5), 4 weeks after the start of the intervention. * Treatment response (minimum decrease of 10 point on the PCL-5 and CAPS-5 scores) at 1, 4 and 12 weeks after the start of the intervention. * Other clinical outcomes 1, 4, and 12 weeks after the start the intervention: * disability (WHO Disability Schedule 2.0; WHO-DAS II), * sleep (Insomnia Severity Index; ISI), * subjective stress (Perceived Stress Scale; PSS), * anxiety symptoms (Beck Anxiety Inventory; BAI), * depressive symptoms (IDS-SR), * suicidal ideation and behaviour (Columbia-Suicide Severity Rating Scale). Trial design: The experimental protocol consists of a placebo-controlled double-blind RCT with 4 face-to-face meetings: * baseline (T0, 2,5 hrs); * post-intervention T1, 8 days after start (1hr); * post-intervention T2, 4 weeks after start (2hr). * post-intervention T3, 12 weeks (2hr). Trial population: 60 adult patients (male/female, 18+ years), with treatment-resistant PTSD (non-response to two evidence-based PTSD treatments, at least one of which is trauma-focused psychotherapy). Intervention Patients are randomized for treatment with the GR antagonist mifepristone (1200 mg/day for 7 days) or matching placebo (daily for 7 days). Study medication will be dispensed during the baseline measurement (T0), and taken once daily for 7 consecutive days. Clinical measurements consist of clinical interviews and questionnaires. During baseline visits a pregnancy test is conducted in woman of child bearing potential (WOCBP), and blood is drawn at T1 to assess mifepristone plasma levels. Ethical considerations relating to the clinical trial including the expected benefit to the individual subject or group of patients represented by the trial subjects as well as the nature and extent of burden and risks: Mifepristone has been clinically used for Cushing's syndrome (anti-glucocorticoid effects) and termination of pregnancy (anti-progesterone effects) for several decades. Mifepristone is generally well-tolerated, and several double-blind studies using the identical duration and dose have shown (7 days, 1200 mg) that the safety profile of mifepristone is comparable to that of placebo treatment, and study dropouts due to side effects were higher for placebo (1.6%) than for mifepristone (1.4%). The most common adverse events (AEs) were nausea, headache, dizziness, and a dry mouth and were comparable between the mifepristone and placebo groups. With regard to mifepristone's progesterone receptor activity and its indication for pregnancy termination, WOCBP who do not agree to use a non-hormonal contraceptive method (condom) during the intervention and up to 1 month after the intervention, are strictly excluded from participating in this study.