MDMA (3,4-methylenedioxymethamphetamine) or 'Ecstasy' was scheduled as an illegal drug in 1986, but since then its recreational use has increased dramatically. This review covers 15 years of research into patterns of use, its acute psychological and physiological effects, and the long-term consequences of repeated use. MDMA is an indirect monoaminergic agonist, stimulating the release and inhibiting the reuptake of serotonin (5-HT) and, to a lesser extent, other neurotransmitters. Single doses of MDMA have been administered to human volunteers in double-blind placebo-controlled trials, although most findings are based upon recreational MDMA users. The 'massive' boost in neurotransmitter activity can generate intense feelings of elation and pleasure, also hyperactivity and hyperthermia. This psychophysiological arousal may be exacerbated by high ambient temperatures, overcrowding, prolonged dancing and other stimulant drugs. Occasionally the 'serotonin syndrome' reactions may prove fatal. In the days after Ecstasy use, around 80% of users report rebound depression and lethargy, due probably to monoaminergic depletion. Dosage escalation and chronic pharmacodynamic tolerance typically occur in regular users. Repeated doses of MDMA cause serotonergic neurotoxicity in laboratory animals, and there is extensive evidence for long-term neuropsychopharmacological damage in humans. Abstinent regular Ecstasy users often display reduced levels of 5-HT, 5-HIAA, tryptophan hydroxylase and serotonin transporter density; functional deficits in learning/memory, higher cognitive processing, sleep, appetite and psychiatric well-being, and, most paradoxically, 'loss of sexual interest/pleasure'. These psychobiological deficits are greatest in heavy Ecstasy users and may reflect serotonergic axonal loss in the higher brain regions, especially the frontal lobes, temporal lobes and hippocampus. These problems seem to remain long after the recreational use of Ecstasy has ceased, suggesting that the neuropharmacological damage may be permament. Copyright 2001 John Wiley & Sons, Ltd.