RATIONALE: 3,4-Methylenedioxymethamphetamine (MDMA, ecstasy) has become a widely used recreational drug among young people. This is of great concern, since MDMA is neurotoxic in animal studies and its use has been associated with psychological distress and a variety of self-reported psychiatric symptoms. However, exploring the origins of psychopathology in ecstasy users is hampered by the frequent polydrug use and by the cross-sectional design of all investigations, so far. OBJECTIVES: The present study combines a cross-sectional with a longitudinal approach to further clarify the impact of the use of other illicit drugs on psychopathological symptoms reported by ecstasy users. METHODS: At baseline, we administered self-rating scales for impulsivity, sensation seeking and general psychological complaints to 60 recreational ecstasy users and 30 matched controls. From the initial sample of ecstasy users, 38 subjects were re-examined 18 months later. RESULTS: At baseline, ecstasy users reported significantly more psychological complaints than controls. However, self-reported psychopathology was mainly associated with regular cannabis use. At follow-up, subjects who had abstained from ecstasy use during the follow-up period did not differ from those reporting continued consumption. In contrast, subjects with regular concomitant cannabis use during the follow-up period reported more anxiety, interpersonal sensitivity and obsessive-compulsive behaviour than cannabis-abstinent users. Finally, higher levels of obsessive-compulsive behaviour, interpersonal sensitivity, depression, anxiety, phobic anxiety and paranoid ideation were significantly correlated with the duration of regular interim cannabis use. CONCLUSIONS: The present findings suggest that self-reported psychopathology in ecstasy users is predominantly attributable to concomitant use of cannabis. Abstinence from cannabis and not ecstasy seems to be a reliable predictor for remission of psychological complaints in ecstasy users. (Author' s abstract)