Enzymes of the Glutathione S-transferase system (GST) modulate the effects of exposure to several cytotoxic and genotoxic agents. The GSTM1 and GSTT1 genes are polymorphic in humans and their deletions have been associated to increased risk of many cancers, including breast cancer. To evaluate the occurrence of homozygous deletions of the GSTM1 and GSTT1 genes in women with sporadic breast cancer and in women without cancer and to compare breast cancer mammographic features between patients with and without these deletions. The study evaluated 100 patients with sporadic breast cancer treated from September 2004 to June 2005 and 169 women without cancer, determining the frequency of the above-mentioned deletions by PCR and calculating the odds ratios and their 95% confidence intervals. Medical files and mammograms of 100 patients with breast cancer were evaluated and correlated with mammographic features such as density, mammographic findings and the BI-RADS classification. These findings were correlated with the genetic deletions by the PR (Prevalence-Ratio) with their respective 95% confidence intervals. The GSTM1 gene was deleted in 40% of the cancers and in 44.4% of controls (OR = 1.20; CI 95% 0.70 - 2.04; p=0.5659) while the GSTT1 gene was deleted in 20% and 19.5%, respectively (OR = 0.73; CI 95% 0.37-1.44; p=0.4124). High mammographic density had been associated with GSTM1 deletion (PR 2.43; CI 1.11 to 4.08). GST deletions were not associated with predominant mammographic findings and the BI-RADS classification. GSTM1 homozygous deletion was associated with high mammographic density.5461-6