Menopausal hormone therapy (HT) is associated with increased breast cancer risk among postmenopausal women. Nuclear receptors are involved in steroid hormone- and xenobiotic-mediated signal transduction playing a crucial role in regulating gene expression. Therefore, variations within these genes may influence HT-associated breast cancer risk. We investigated 3,149 postmenopausal breast cancer patients and 5,489 controls from 2 German population-based case-control studies. Thirty-three polymorphisms selected on the basis of known or putative functional relevance located in ESR1, ESR2, PGR, PXR and AR were genotyped. Conditional logistic regression was used to assess multiplicative statistical interaction between polymorphisms and duration of estrogen-progestagen therapy and of estrogen monotherapy with regard to breast cancer risk assuming log-additive and codominant modes of inheritance. We observed an increased risk for women carrying short AR_(CAG) alleles of <22 repeats associated with combined estrogen-progestagen therapy compared with those with long alleles (> or =22 repeats) (p(interaction) = 0.03). Additionally, risk associated with combination therapy use was significantly modified by 2 PXR polymorphisms with reduction of risk effects in carriers of the minor PXR_rs6785049_G and PXR_rs1054191_A alleles (p(interaction) = 0.04 and 0.05, respectively). Variants in both ESR1 and ESR2 modified risk associated with estrogen monotherapy use. Higher risk were observed in homozygotes for the major ESR1_rs910416_T allele (p(interaction) < 0.01) and in homozygotes for the minor ESR2_rs1271572_T, major ESR2_rs4986938_G and minor ESR2_rs928554_G alleles (p(interaction) = 0.02, 0.05, 0.02, respectively). Risk effect modification by ESR1_rs910416 and AR_(CAG)n polymorphisms remained significant after correction for multiple testing. We conclude that genetic variants in nuclear receptor genes may modify HT-associated postmenopausal breast cancer risk.