ObjectiveEpidemiological studies have shown evidence of the effect of sex hormones in the pathogenesis of breast cancer, and have suggested a relationship of the disease with variations in genes involved in estrogen synthesis and/or metabolism. The aim of the present study was to evaluate the association between theCYP3A4*1Bgene polymorphism (rs2740574) and the risk of developing breast cancer.MethodsIn the present case-control study, the frequency of theCYP3A4*1Bgene polymorphism was determined in 148 women with breast cancer and in 245 women without the disease. The DNA of the participants was extracted from plasma samples, and the gene was amplified by polymerase chain reaction. The presence of the polymorphism was determined using restriction enzymes.ResultsAfter adjusting for confounding variables, we have found that the polymorphism was not associated with the occurrence of breast cancer (odds ratio = 1.151; 95% confidence interval: 0.714–1.856;p= 0.564). We have also found no association with the presence of hormone receptors, with human epidermal growth factor receptor 2 (HER2) overexpression, or with the rate of tumor cell proliferation.ConclusionWe have not observed a relationship between theCYP3A4*1Bgene polymorphism and the occurrence of breast cancer.