due to the conflicting results regarding the association between breast cancer and the gstm1 null mutation, our aim was to research this association in a brazilian population and correlations with smoking, 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 single gene gstm1 polymorphisms do not confer a substantial risk of breast cancer to its carriers. furthermore, in this study no correlation was found between gsts and smoking, reproductive history and several clinical pathologies with respect to cancer risk.