Estrogen-related receptor a (ERRa), a member of the nuclear receptor superfamily, is closely related to the estrogen receptors (ERa and ERß). The ERRa gene is estrogen responsive in several mouse tissues and cell lines, and a multiple hormone response element (MHRE) in the promoter is an important regulatory region for estrogen-induced ERRa gene expression. ERRa was recently shown to be a negative prognostic factor for breast cancer survival, with its expression being highest in cancer cells lacking functional ERa. The contribution of ERRa in breast cancer progression remains unknown but may have important clinical implications. In this study, we investigated ERRa gene expression and chromatin structural changes under the influence of 17ß-estradiol (E2) in both ER-positive MCF-7 and ER-negative SKBR3 breast cancer cells. We mapped the nucleosome positions of the ERRa promoter around the MHRE region and found that the MHRE resides within a single nucleosome. Local chromatin structure of the MHRE exhibited increased restriction enzyme hypersensitivity and enhanced histone H3 and H4 acetylation upon estrogen treatment. Interestingly, estrogen-induced chromatin structural changes could be repressed by estrogen antagonist ICI 182 780 in MCF-7 cells yet were enhanced in SKBR3 cells. We demonstrated, using ChIP assays, that E2 induces ERRa gene expression in MCF-7 cells through active recruitment of coactivators and release of corepressors when ERRa and AP1 bind and ERa is tethered to the MHRE. We also found that this estrogen effect requires the MAPK signaling pathway in both cell lines.