3,4-Methylenedioxymethamphetamine (MDMA; "ecstasy") is a selective serotonin (5-HT) neurotoxin in animals. There is now preliminary evidence in humans of 5-HT deficits associated with extensive use of MDMA. In order to begin to describe the cognitive and mood effects of chronic MDMA use, nine individuals with extensive MDMA use histories were studied. Despite the absence of memory deficits on clinical examination, a pattern of mild-to-moderate impairment was observed on both the Initial and Delayed Paragraph Tests of the Wechsler Memory Scale; eight of the subjects had at least mild impairment on at least one test in the neuropsychological battery. Despite previously reported suggestive evidence of 5-HT deficit in this group, none reported depressed mood or met clinical criteria for an affective disorder at the time of testing. These preliminary findings raise concern about possible detrimental effects of MDMA use on neuropsychological function for future systematic study and they highlight important issues regarding the effects of 5-HT deficits on cognitive function and mood regulation.