Abstract Purpose: In breast cancer, the presence of estrogen receptor α (ER) denotes a better prognosis and response to antiestrogen therapy. Lack of ERα correlates with overexpression of epidermal growth factor receptor or c-erbB-2. We have shown that hyperactivation of mitogen-activated protein kinase (MAPK) directly represses ERα expression in a reversible manner. In this study, we determine if inhibition of MAPK in established ERα− breast cancer cell lines and tumors results in reexpression of ERα, and further, if reexpression of ERα in these ERα− tumors and cell lines could restore antiestrogen responses. Experimental Design: Established ERα− breast cancer cell lines, ERα− breast tumors, and tumor cell cultures obtained from ERα− tumors were used in this study. Inhibition of hyperactive MAPK was accomplished via the MAPK/ERK kinase 1/2 inhibitor U0126 or via upstream inhibition with Iressa or Herceptin. Western blotting or reverse transcription-PCR for ERα was used to assess the reexpression of ERα in cells treated with U0126. Growth assays with WST-1 were done to assess restoration of antiestrogen sensitivity in these cells. Results: Inhibition of MAPK activity in ERα− breast cancer cell lines results in reexpression of ERα; upstream inhibition via targeting epidermal growth factor receptor or c-erbB-2 is equally effective. Importantly, this reexpressed ERα can now mediate an antiestrogen response in a subset of these ERα− breast cancer cell lines. Treatment of ERα− tumor specimens with MAPK inhibitors results in restoration of ERα mRNA, and similarly in epithelial cultures from ERα− tumors, MAPK inhibition restores both ERα protein and antiestrogen response. Conclusions: These data show both the possibility of restoring ERα expression and antiestrogen responses in ERα− breast cancer and suggest that there exist ERα− breast cancer patients who would benefit from a combined MAPK inhibition/hormonal therapy.