Two persons are described who demonstrated prolonged neuropsychiatric syndromes after the ingestion of large doses of (+-)-3,4-methylenedioxymethamphetamine (MDMA), a recreationally used amphetamine analog. These cases suggest that MDMA, known to be neurotoxic to serotonin neurons in several experimental animals, may also produce untoward effects in humans. In addition, they provide evidence that ingestion of large doses of MDMA can produce lasting adverse functional consequences in vulnerable persons.