3,4 Methylenedioxymethamphetamine (MDMA or ecstasy) has become a major drug of abuse over the last decade. It produces a mixture of systemic and neuropsychological effect. Animal studies show a range of short- and long-term toxic effects, both systemically and neurochemically. In humans, toxicity and death due to the drug have been attributed to a variety of causes, with 'idiosyncratic', or non-dose-related, reactions often cited. It has recently been established that MDMA is metabolized via the cythocrome P450 enzyme, debrisoquine hydroxylase. This enzyme is coded by the gene CYP2D6. This gene contains mutations which effect the function of the enzyme, and individuals homozygous for these mutations are known as poor metabolizers. Between 3 and 1O% of the Caucasian population are thus affected, and therefore may be less able to metabolize MDMA. In this paper we examine the hypothesis that individuals selected on the basis of having had an adverse reaction to MDMA will be more likely than the general population to have homozygous mutations at CYP2D6. We obtained retrospectively seven cases of toxicity or death thought to be due to MDMA. DNA was extracted from these patients, and their genotype ascertained. None of this small sample was shown to be homozygous for the mutation at CYP2D6. Three possible explanations are offered for these results. (1) The non-dose-related nature of MDMA toxicity may be due, either alone or in combination, to contaminants in the drug, or ambient environmental/physiological factors. (2) Our genotyping methods may have missed one of the rare additional mutations which effect gene function at CYP2D6. (3) Our study sample may be too small to demonstrate a statistically significant result. A larger study is currently under way.