Rationale: (±) 3,4-Methylenedioxymethamphetamine (MDMA, “ecstasy”) is an amphetamine analog and drug of abuse. In animals, MDMA damages brain serotonin (5-HT) neurons at doses that overlap with those used recreationally by some humans. To date, few functional sequelae of MDMA-induced 5-HT damage have been identified. Objective: Since serotonin is thought to be involved in cognitive processes, and since previous studies have reported verbal and visual memory deficits in MDMA users, the present study sought to determine whether other cognitive processes are influenced by previous exposure to MDMA. Methods: Twenty-two MDMA users who had not used MDMA for at least 3 weeks and 23 control subjects were tested repeatedly with a computerized cognitive performance assessment battery while participating in a 5-day controlled inpatient study. Cerebrospinal fluid (CSF) measures of monoamine metabolites were also collected as an index of brain monoaminergic function. Results:MDMA users and controls were found to perform similarly on several cognitive tasks. However, MDMA subjects had significant performance deficits on a sustained attention task requiring arithmetic calculations, a task requiring complex attention and incidental learning, a task requiring short term memory and a task of semantic recognition and verbal reasoning. MDMA users also had significant selective decreases in CSF 5-HIAA. Conclusions: The present CSF data provide further evidence that MDMA is neurotoxic to brain 5-HT neurons in humans, and the behavioral data suggest that brain 5-HT injury is associated with subtle, but significant, cognitive deficits.