Long-term estrogen exposure and family history of breast cancer are the two factors that are most consistently found to be associated with breast cancer risk. Sequence variants in genes involved in estrogen synthesis, metabolism, and signal transduction may account, in part, for this observation. Using data and DNA samples from the Shanghai Breast Cancer Study, we tested the hypothesis that sequence variants of the estrogen receptor beta gene (ESR2) may be associated with increased risk for breast cancer, particularly among women who have a high level and long-term endogenous estrogen exposure. Direct sequencing of the ESR2 gene among 30 Chinese women revealed eight sequence variants. Association analysis of six common sequence variants in 1134 cases and 1235 controls provided evidence for positive associations between breast cancer risk and two single nucleotide polymorphisms (SNPs), [C(14206)T and C(33390)G], among postmenopausal women. Evidence of a stronger association was found for SNP [C(33390)G] among women with a long duration (> or =34 years) of menstruation (odds ratio, 2.37; 95% confidence interval, 1.18-4.77). A potential synergistic effect between SNP [C(33390)G] and several steroid sex hormones was observed, and a 3-4-fold elevated risk of breast cancer was found among women with a CG or GG genotype in SNP [C(33390)G] combined with a high level of steroid sex hormone or a low level of sex hormone binding globulin. Our results are consistent with the hypothesis of a joint effect of estrogen receptor beta sequence variants and endogenous estrogen exposure on breast cancer risk.