Glutathione S-transferases GSTM1, GSTT1, and GSTP1 are phase II biotransformation enzymes that function on detoxification of a wide range of exogenous agents including carcinogens. It has been shown that genetic variations in these genes play an important role in determining the response of an individual to environmental carcinogens. Some studies revealed a statistically significant association between the polymorphisms in the genes encoding GST enzymes and some cancers, although contrary reports exist. In this study, the association between polymorphisms in these genes and primary brain tumor incidence was investigated in 228 Turkish individuals (75 patients with primary brain tumor and 153 controls). The prevalence of GSTM1 null genotype in the case group was 43%, compared to 24% in the control group, giving an odds ratio (OR) of 2.33 (95% confidence interval CI=1.24-4.39). No association was observed between the GSTT1 or GSTP1 Ile105Val polymorphism and brain tumor incidence. Polymorphisms in GSTM1, GSTT1, and GSTP1 did not show association with histopathologic type of brain tumor (glioma or meningioma). Analysis of the polymorphisms in the studied genes and smoking status of the brain tumor patients revealed no statistically significant association. The presented data clearly suggest a relation between brain tumor incidence with GSTM1 null genotype but not with GSTT1 or GSTP1 gene variants.