PURPOSE: To evaluate the potential association between the GSTP1 genotype and the development of breast cancer, a hospital based case-control study was conducted in South Korea. MATERIALS AND METGODS: The study population consisted of 171 histologically confirmed incidents of breast cancer cases, and 171 age-matched controls with no present, or previous, history of cancer. A PCR method was used for the genotyping analyses, and statistical evaluation was performed by an unconditional logistic regression model. RESULTS: No association was observed in the study subjects, or the premenopausal women group with GSTP1 Val allele. However, postmenopausal women with GSTP1 Val allele had a reduced risk of breast cancer (OR=0.3, 95% CI=0.1~0.7). When the data were stratified, by the known risk factors of breast cancer, a significant interaction was observed between the GSTP1 genotype and alcohol consumption (p for interaction = 0.01); women with GSTP1 Val allele, that drank regularly, had a 3.0-fold increased risk of breast cancer (95% CI=1.1~7.9), whereas women with GSTP1 Val allele, that never drink, had protective effects (OR=0.4, 95% CI=0.2~0.8). CONCLUSION: Our findings suggest that GSTP1 Ile105Val polymorphism influences the individual susceptibility to breast cancer, and that this effect may be modified by alcohol consumption.