Hormone therapy, estrogen plus progestin (E+P) particularly, is associated with increased risk of breast cancer. Functionally relevant polymorphisms in genes involved in sex hormone metabolism may alter exposure to exogenous sex hormones and affect risk of postmenopausal breast cancer. We evaluated associations of common polymorphisms in genes involved in estrogen and/or progesterone metabolism, E+P use, and their interactions with breast cancer risk in a case-control study of postmenopausal women (324 cases; 651 controls) nested within the VITAL cohort. None of the polymorphisms studied was, by itself, statistically significantly associated with breast cancer risk. E+P use was significantly associated with increased breast cancer risk (> or =10 years versus never; odds ratio, 1.9; 95% confidence interval, 1.3-2.8; P(trend) = 0.0002). Statistically significant interactions between CYP1A1 Ile(462)Val (P(interaction) = 0.04), CYP1A1 MspI (P(interaction) = 0.003), CYP1B1 Val(432)Leu (P(interaction) = 0.007), CYP1B1 Asn(453)Ser (P(interaction) = 0.04) and PGR Val(660)Leu (P(interaction) = 0.01), and E+P use were observed. The increased risk of breast cancer associated with E+P use was greater among women with at least one rare allele of the CYP1A1 Ile(462)Val, CYP1A1 MspI, CYP1B1 Asn(453)Ser, and PGR Val(660)Leu polymorphisms than among women homozygous for the common allele of these polymorphisms. Risk of breast cancer increased little with increasing years of E+P use among women with at least one CYP1B1 Val(432) allele; a large increase in risk was seen among women homozygous for CYP1B1 Leu(432). Our results support the hypothesis that specific polymorphisms in genes involved in sex hormone metabolism may modify the effect of E+P use on breast cancer risk.