The NAT2, GSTM1 and GSTT1 genes are known candidate cancer susceptibility markers and have been investigated in breast cancer susceptibility with conflicting results. We conducted a case-control study to investigate the role of NAT2, GSTM1 and GSTT1 in premenopausal breast cancer. Women with the GSTT1 null genotype were found to have a significant 3.15-fold increased risk of breast cancer (95% CI = 1.7-5.8), while GSTM1 and NAT2 genotypes were not associated with breast cancer risk. Our results suggest that the GSTT1 null genotype may play a role in early onset breast cancer.