Due to the conflicting results regarding the association between breast cancerand the GSTM1 null mutation, our aim was to research this associationin a Brazilian population and correlations withsmoking, reproductive history and several clinical pathologies. A case-control study was performed on 105 women with breast cancer and 278 controls. Extraction of DNA was accomplished according to the protocol of the GFX® kit and polymorphism analysis by the PCR technique. The control and experimental groups were compared and statistical analysis assessed by X² or Fisher's exact test. The deletion in the GSTM1 gene in the breast cancer group had a prevalence of 32 (30.4%) individuals with the presence of null mutation. In the control group, the null mutation was present in 104 (37.4%) women. Upon comparison of the two groups, no statistically significant difference of the GSTM1 gene was observed, with an odds ratio (OR) of 0.74, 95%, confidence interval (CI) 0.45 - 1.20, p = 0.277. The results conclusively show that singlegene GSTM1 polymorphisms do not confer a substantial risk of breastcancer to its carriers. Furthermore, in this study no correlation was found between GSTs andsmoking, reproductive history and several clinical pathologies with respect to cancer risk.