The isoenzyme Mu of glutathione S-transferase (GSTM1) is dominantly inherited, and the prevalence of this isoenzyme in the population is about 60%. The lack of GSTM1 has been linked with cancer risk. The frequency of the phenotypes of this isoenzyme in melanoma (MM) patients (n = 197) is reported here. A significantly higher proportion of individuals in the control group (n = 147) had measurable GSTM1 than MM patients (59.1% vs 42%, P = 0.002); there was a higher proportion of positive phenotypes in general among women than among men. Odds ratio analysis indicated that individuals with this polymorphic variant have an approximately 2-fold risk of developing these cancers. GSTM1 phenotype distribution depends on age, smoking habit and tumour pathology. A group of MM patients with dysplastic naevi was also studied.