Inclusion Criteria:

• Diagnosed with PTSD as a result of crime victimization or as a result of combat if symptoms have lasted for no longer than five years.
• Very little or no improvement in PTSD signs or symptoms after three months' treatment with a selective serotonin uptake inhibitor (SSRI) and little or no improvement after six months' treatment with any form of psychotherapy for which there there is evidence from a controlled trial that this type of therapy efficacious (good for treating) PTSD.
• Must be willing to attend all sessions, and to take MDMA or placebo.
• If you are seeing another psychotherapist, you must be willing to give the principal investigator permission to communicate with him or her.
• Must be willing to stop taking any psychiatric medications during the study period. Drugs will be tapered in an appropriate fashion to avoid withdrawal effects. Participants must not start any new medications until after the evaluation session, which will occur 2 months after the second experimental session.
• Must be willing to remain overnight at the clinic after each experimental session until the non-drug session occurring the next morning. An attendant will be present to assist with personal needs if requested and offer dinner and breakfast. The attendant will be a registered nurse (RN) and of the same sex as the participant, and he or she will be trained for assisting in this study.
• Participants must be willing to be contacted via telephone on a daily basis by one of the investigators for a week after each experimental session.
• Female participants of childbearing potential must have a negative pregnancy test and must agree to use an effective form of birth control.
• Participants must be literate. They must be proficient in reading documents written in English.

Exclusion Criteria:

• People with a non-crime or non-war related traumatic event that makes a significant contributor to their PTSD symptoms, as assessed by the CAPS.
• Women who are pregnant or nursing, or who are able to get pregnant and are not practicing an effective means of birth control.
• People with a history of or current primary psychotic disorder or bipolar affective disorder type 1.
• People with dissociative identity disorder or an eating disorder with active purging, or borderline personality disorder.
• People with hypertension, peripheral vascular disease, hepatic disease (with or without abnormal liver enzymes), or history of hyponatremia or hyperthermia.
• People with evidence or history of significant hematological, endocrine, cerebrovascular, cardiovascular, coronary, pulmonary, renal, gastrointestinal, immunocompromising, or neurological disease, including seizure disorder. (Participants with hypothyroidism who are on adequate and stable thyroid replacement will not be excluded).
• Weighing less than 50 kg (110 lb) or more than 105 kg (230 lb).
• People who reported they used "Ecstasy" (material represented as MDMA) more than 5 times or at any time within the previous 6 months.
• People who would present a serious suicide risk or who are likely to require hospitalization during the course of the study.
• People requiring ongoing concomitant therapy with a psychotropic drug.
• People diagnosed with substance abuse or dependence for any substance save caffeine or nicotine in the past 60 days.
• Any person who is not able to give adequate informed consent.