Estrogen receptor (ER)-alpha and ER-beta function as transcription factors, and both interact with nuclear regulatory proteins to enhance or inhibit transcription. We hypothesized that coregulators are expressed in breast cancer and may be differentially recruited by ERs in the presence of estrogen and tamoxifen. ER-beta was found to be expressed more frequently in node-negative patients (P < 0.05). Expression of steroid receptor coactivator-1 (SRC-1) was associated with nodal positivity (P < 0.05) and resistance to endocrine treatment (P < 0.001). The spatial coexpression of ER-alpha, ER-beta, and the coregulatory proteins was established using immunofluorescence. In both cell lines (MCF-7 and T47D) and in primary breast cancer cell cultures, beta-estradiol up-regulated ER-beta and coregulator protein expression and increased ER-alpha/ER-beta interaction with the estrogen response element (ERE). 4- Hydroxy-tamoxifen (4-OHT) increased ER-alpha and silencing mediator for retinoid and thyroid receptors (SMRT) expression and increased ER-ERE binding. SRC-1 and SMRT were identified at the ER-ERE complex, and interactions between ER isoforms and coregulatory proteins were determined using immunoprecipitation. Both ER-alpha and ER-beta preferentially bound SRC-1 in the presence of beta-estradiol. Conversely, in cells treated with 4-OHT, ER-alpha and ER-beta bound SMRT. Differential recruitment of SRC-1 and SMRT by ER-alpha and ER-beta in the presence of beta-estradiol and 4-OHT may be central to the response of the tumor to endocrine treatment.