This study was undertaken to examine if glutathione S-transferase (GST) M1, M3, P1, and T1 genotypes affected breast cancer risk in Finnish women. The study population consisted of 483 incident breast cancer cases and 482 healthy population controls. Genotyping analyses were performed by PCR-based methods, and odds ratios (ORs) and 95% confidence intervals (CIs) were calculated by unconditional logistic regression adjusting for known or suspected risk factors for breast cancer. When the genes were studied separately, the only significant finding was between GSTM1 null genotype and postmenopausal breast cancer risk (OR, 1.49; 95% CI, 1.03-2.15). Conversely, when the potential combined effects of the at-risk genotypes were examined, significant associations were observed only among premenopausal women. Although only a moderate risk of breast cancer was seen for premenopausal women concurrently carrying the GSTM3*B allele containing genotypes and the GSTP1 Ile/ Ile genotype (OR, 2.07; 95% CI, 1.02-4.18), the risk rose steeply if they simultaneously lacked the GSTT1 gene (OR, 9.93, 95% CI, 1.10-90.0). A borderline significant increase in the risk of breast cancer was also seen for premenopausal women with the combination of GSTM1 null, GSTP1 Ile/Ile, and GSTT1 null genotypes (OR, 3.96; 95% CI, 0.99-15.8). Our findings support the view that GST genotypes contribute to the individual breast cancer risk, especially in certain combinations.