This study will investigate the effects of an additional 3,4-methylenedioxymethamine (MDMA)-assisted psychotherapy session in up to three participants with PTSD who relapsed after their PTSD symptoms had significantly decreased during MAPS' initial U.S. Phase 2 trial testing the use of MDMA-assisted psychotherapy in subjects with chronic, treatment-resistant posttraumatic stress disorder PTSD). This new protocol will consist of a single, full-dose open-label session of MDMA-assisted psychotherapy, along with associated non-drug preparation and integrative psychotherapy sessions.