Patients with estrogen receptor (ER)-positive breast cancers have a better prognosis than those with ER-negative breast cancers, but often have low sensitivity to chemotherapy and a limited survival benefit. We have previously shown a combination effect of taxanes and fulvestrant and suggested that this treatment may be useful for ER-positive breast cancer. In this study, we evaluated the effects of combinations of hormone drugs and chemotherapeutic agents. In vitro, the effects of combinations of five chemotherapeutic agents (doxorubicin, paclitaxel, docetaxel, vinorelbine, and 5-fluorouracil) and three hormone drugs (fulvestrant, tamoxifen, and 4-hydroxytamoxifen) were examined in ER-positive breast cancer cell lines using CalcuSyn software. Changes in chemoresistant factors such as Bcl2, multidrug resistance-associated protein 1, and microtubule-associated protein tau were also examined after exposure of the cells to hormone drugs. In vivo, tumor sizes in mice were evaluated after treatment with docetaxel or doxorubicin alone, fulvestrant alone, and combinations of these agents. Combination treatment with fulvestrant and all five chemotherapeutic agents in vitro showed synergistic effects. In contrast, tamoxifen showed an antagonistic effect with all the chemotherapeutic agents. 4-Hydroxytamoxifen showed an antagonistic effect with doxorubicin and 5-fluorouracil, but a synergistic effect with taxanes and vinorelbine. Regarding chemoresistant factors, Bcl2 and microtubule-associated protein tau were downregulated by fulvestrant. In vivo, a combination of fulvestrant and docetaxel had a synergistic effect on tumor growth, but fulvestrant and doxorubicin did not show this effect. In conclusion, fulvestrant showed good compatibility with all the evaluated chemotherapeutic agents, and especially with docetaxel, in vitro and in vivo. (Cancer Sci 2011; 102: 2038-2042 T he malignant grade of estrogen receptor (ER)-positive breast cancer is generally low, thus patients with this cancer have a better prognosis than those with ER-negative breast cancer. However, ER-positive breast cancers have low sensitivity to chemotherapy and the survival benefit of chemotherapy is limited.(1-3) Estrogen receptors play a crucial role in the development and progression of breast cancers and modulate many genes including of chemoresistant factors. The low sensitivity to chemotherapy may be caused by ER itself or by ER modulation of the levels of factors that cause resistance to chemotherapy. (4)(5)(6)(7)(8)(9)(10)(11) Tamoxifen has significant efficacy for ER-positive breast cancer. However, the mechanism is not completely understood. Tamoxifen is metabolized and the metabolites (4-hydroxytamoxifen and 4-hydroxy-N-desmethyltamoxifen) have different affinities for ER and exert a variety of agonist and antagonist properties.(1,12-14) Fulvestrant, a selective ER inhibitor, is an active hormone drug for advanced breast cancer, and a randomized phase II comparative study of a fulvestrant high-dose regimen for advanced hormone receptor-pos...