The isolation of full-length cDNAs of naturally occurring GSTP1 gene variants, and the demonstration that these alleles are distributed in the normal population, have provided conclusive evidence that the human GSTP1 gene locus is polymorphic and that specific GSTP1 alleles may be associated with different risk for cancers or other diseases. Recent data have indicated that the different GSTP1 alleles encode proteins with different enzymatic activities against carcinogens. In this case-control study, we examined the effect of the GSTP1 genetic polymorphism and its interaction with other factors to determine breast cancer risk. GSTP1 and GSTM1 genotypes of 220 breast cancer patients and 196 controls, all residents of western France, were examined. Data on menopausal status and family cancer history were obtained from 195 patients and 147 controls. Exons 5 and 6 of the GSTP1 gene, which contain the polymorphic nucleotide transitions, were analyzed by DNA polymerase chain reaction-restriction fragment length polymorphism to distinguish between the GSTP1 alleles. In the control population, GSTP1 allelic frequencies were 64.3%, 26.0% and 9.7%, respectively, for GSTP1*A, GSTP1*B and GSTP1*C. In the breast cancer patients, the frequencies were 67.9% for GSTP1*A, 26.8% for GSTP1*B and 5.3% for GSTP1*C. In multivariate analysis, breast cancer risk increased by 7.7-fold (p < 0.001) in women with a family history of cancers and 2.18-fold (p = 0.026) in non-GSTP1*C individuals. GSTM1 genotypes did not emerge as risk factor. Our results show that in addition to well-known risk factors, in particular, a family history of cancer, GSTP1 allelopolymorphism is a significant modifier of breast cancer risk. The results also suggest a protective role against breast cancer for the GSTP1*C allele.