This study is designed to provide information on whether psychotherapy ("talk therapy") combined with the drug MDMA is safe and helpful for subjects with posttraumatic stress disorder (PTSD). The study will compare the effects of a low, a medium and a full dose of MDMA on symptoms of PTSD in 24 veterans, firefighters or police officers. MDMA dose will be assigned at random, and the investigators and the subject will not know the dose given. The researchers will also investigate depression symptoms. The researchers believe that the full dose of MDMA will produce a greater reduction in PTSD symptoms than the two lower doses.