Polycyclic aromatic hydrocarbons, possible human breast carcinogens, are metabolized by cytochrome P4501A1 (CYP1A1) and glutathione S-transferase (GSTM1). A CYP1A1 polymorphism (isoleucine to valine substitution in exon 7) or the null allele for GSTM1 may affect the mutagenic potential of polycyclic aromatic hydrocarbons. We examined polymorphisms in GSTM1 and CYP1A1 in relation to breast cancer risk. Included were 216 postmenopausal Caucasian women with incident breast cancer and 282 community controls. DNA analyses suggested no increased breast cancer risk with the null GSTM1 genotype [odds ratio (OR) = 1.10; CI, 0.73-1.64], although there was some indication that the null genotype was associated with risk among the youngest postmenopausal women (OR = 2.44; CI, 0.89-6.64). Slightly elevated risk was associated with the CYP1A1 polymorphism (OR = 1.61; CI, 0.94-2.75) and was highest for those who smoked up to 29 pack-years (OR = 5.22; CI, 1.16-23.56). Statistical power to detect an effect may be limited by small numbers, and larger sample sizes would be required to corroborate these suggestive findings.