Background. Use of MDMA (ecstasy), a serotonin neurotoxin, has been associated with memory impairment and psychological dysfunction. This study examined cognitive functioning in abstinent MDMA users and MDMA-naïve controls. Method. Participants completed measures of intelligence, motor function, attention, memory span, verbal fluency, immediate and delayed verbal memory, and working memory. They were also assessed for the presence of psychopathology. In addition to comparing cognitive function in MDMA users relative to controls, the possibility that clinically dysfunctional MDMA use increases the risk of cognitive impairment was examined. Results. MDMA users exhibited relative deficits in mnemonic and executive functions. Additionally, users that met DSM-IV substance use disorder criteria for lifetime MDMA abuse or dependence exhibited a number of additional deficits relative to those who did not meet these criteria. Conclusion. These findings suggest that clinically dysfunctional, rather than purely recreational, MDMA use is associated with cognitive impairment. Future research studies of diverse samples of users may shed light on the mechanisms that underlie these differences.