This study will consist of a single session of MDMA-assisted psychotherapy with a full dose of MDMA for people who took part in a study of MDMA-assisted psychotherapy in people with posttraumatic stress disorder (PTSD) whose PTSD symptoms returned to higher levels at least a year after the first MDMA sessions. The single session will be performed by the same pair of therapists who performed the sessions in the first study. People will have a preparatory session, the MDMA-assisted session and three non-drug sessions afterward. Their PTSD symptoms and symptoms of depression will be measured at the start of this study and two and 12 months after the session.