AIMS: This study was established to determine if users of ecstasy had greater tooth wear than a comparison group of non-users. DESIGN: The study involved a direct comparison between two groups of young people gathered by a peer information network and divided by the response to the question "Do you take ecstasy?" SETTING: The information network was established at the Maryland Centre in Liverpool (UK). Besides offering advice on drug and sex-related problems this centre distributes free condoms and is therefore visited by many young people who would not consider themselves to have these problems. PARTICIPANTS: There were 30 users and 28 non-users with no significant difference in the gender ratio. Age was not recorded to maintain absolute confidentiality, but all the participants appeared to be of student age. MEASUREMENTS: Tooth wear was measured using the index of Smith & Knight. The social and drug-taking profile of the two groups was ascertained by questionnaire. FINDINGS: It was found that the mean (+/- STD) tooth wear score for the back teeth was 0.96 +/- 0.16 in the users compared with 0.12 +/- 0.08 for the comparison group. These values were significantly different (p < 0.001). The values for the front teeth were not significantly different. Many users were aware of clenching their teeth 12 or even 24 hours after taking the drug. CONCLUSION: Taking ecstasy results in an increased likelihood of tooth wear on the back teeth. This is likely to result from clenching the teeth in the acidic environment caused by carbonated (fizzy) drinks.