Some studies have reported an association of the GSTM1-null genotype with the risk of smoking-related cancers, such as lung, bladder, and colon cancer. Because the risk of anal cancer is strongly associated with a history of cigarette smoking, we examined whether the GSTM1-null genotype is a susceptibility marker for anal cancer. We obtained peripheral blood specimens from residents of western Washington who were diagnosed with squamous or transitional cell tumor of the anus between April 1991 and June 1994. Eligible for inclusion were persons 18-74 years of age, with either invasive or in situ lesions. Specimens were also obtained from controls identified via random-digit dialing of western Washington households. We determined GSTM1 genotypes of 71 cases and 360 controls by PCR using primer pair 5'-AACTCCCTGAAAAGCTAAAGC-3' and 5'-GTTGGGCTCAAATATACGGTGG-3'. The frequency of the GSTM1-null genotype in controls was approximately 57%; this differed little in relation to age, sex and smoking status. The incidence of anal cancer appeared to be reduced in persons with the GSTM1-null genotype; only 39.4% of cases had this genotype (age-adjusted odds ratio = 0.5, 95% confidence interval = 0.3-0.9). This inverse association was restricted to persons who had ever smoked cigarettes and was present in both women and men (and in the latter, in both those who had and did not have a male sexual partner). Our data strongly suggest that persons with the GSTM1-null genotype are not at increased risk of anal cancer, and may well be at a decreased risk.