Recent preclinical evidence suggests that repeated exposure to 3, 4-methylenedioxy-methamphetamine (MDMA; "ecstasy") produces long-term reductions in serotonin (5-HT) levels. 5-HT has been implicated in the regulation of mood, anxiety, aggression, impulsivity, and cognition. Accordingly, in the first of two separate studies, these variables were investigated in three groups: (1) MDMA group--recreational ecstasy users (who also used other illicit substances); (2) polydrug controls--who had never taken ecstasy, but otherwise had drug histories and personal characteristics similar to the ecstasy users; and (3) nondrug controls--who had never used illicit drugs, but had similar personal characteristics. All participants completed mood (Likert) scales, personality questionnaires (which included the impulsiveness, venturesomeness and empathy questionnaire-IVE), spatial span and "Tower of London" (TOL) tests, and a behavioural measure of impulsivity, the matching familiar figures test (MFF20). There were no group differences in mood, anxiety, anger/hostility, and cognitive performance, but the MDMA group committed significantly more errors in the MFF20. Subsequently, a larger sample of participants were administered mood (the General Health Questionnaire or GHQ) and personality (IVE) questionnaires before the administration of a TOL test, followed by the MFF20, and a second TOL test. Although there were no group differences in TOL performance, ecstasy users were again found to commit more errors in the MFF20 than polydrug users. Furthermore, the GHQ and IVE scores of the ecstasy users in the second study indicated, respectively, that they were more psychologically disturbed and impulsive than nondrug controls. The combined data from the two studies indicated that ecstasy users exhibited elevated impulsivity on both self-report and behavioral measures and that those who had taken the most ecstasy had the most elevated trait impulsiveness scores. These findings are consistent with previous evidence that elevated levels of impulsivity in humans are associated with reduced levels of serotonergic function.