 Posttraumatic stress disorder PTSD occurs after experiencing a traumatic event or events. PTSD is a public health problem that causes a great deal of suffering. This study will examine whether two six to eight-hour long sessions of 3,4-methylenedioxymethamphetamine MDMA-assisted psychotherapy scheduled three to five weeks apart are safe, and whether combining a fully therapeutic dose of MDMA with psychotherapy, compared with a low active placebo dose of MDMA, will reduce PTSD symptoms, with symptoms measured four times, twice during the study, and during two follow-up assessments six and twelve months after the second experimental session. People who received the active placebo dose of MDMA can then take part in an open label study continuation, with the participant receiving a fully active dose of MDMA on two more six to eight hour-long psychotherapy sessions. Open-label means that the participants and the researchers know that the participant will receive the fully active dose of MDMA. People who receive the full dose of MDMA, and anyone who received low-dose MDMA and does not undergo the open-label study continuation will have PTSD symptoms measured six and twelve months after the second fully active or low dose MDMA session. People who take part in the open label study continuation have their PTSD symptoms checked six and 12 months after the second open label MDMA-assisted session. 