The ERalpha signaling pathway is one of the most important and most studied pathways in human breast cancer, yet numerous questions still exist such as how hormonally responsive cancers progress to a more aggressive and hormonally independent phenotype. We have noted that human breast cancers exhibit a strong direct correlation between ERalpha and E-cadherin expression by immunohistochemistry, suggesting that ERalpha signaling might regulate E-cadherin and implying that this regulation might influence epithelial-mesenchymal transition (EMT) and tumor progression. To investigate this hypothesis and the mechanisms behind it, we studied the effects of ERalpha signaling in ERalpha-transfected ERalpha-negative breast carcinoma cell lines, the MDA-MB-468 and the MDA-MB-231 and the effects of ERalpha knockdown in naturally expressing ERalpha-positive lines, MCF-7 and T47D. When ERalpha was overexpressed in the ERalpha-negative lines, 17beta-estradiol (E2) decreased slug and increased E-cadherin. Clones maximally exhibiting these changes grew more in clumps and became less invasive in Matrigel. When ERalpha was knocked down in the ERalpha-positive lines, slug increased, E-cadherin decreased, cells became spindly and exhibited increased Matrigel invasion. ERalpha signaling decreased slug expression by two different mechanisms: directly, by repression of slug transcription by the formation of a corepressor complex of ligand-activated ERalpha, HDAC inhibitor (HDAC1), and nuclear receptor corepressor (N-CoR) that bound the slug promoter in three half-site estrogen response elements (EREs); indirectly by phosphorylation and inactivation of GSK-3beta through phosphoinositide 3-kinase (PI3K)/protein kinase B (Akt). The GSK-3beta inactivation, in turn, repressed slug expression and increased E-cadherin. In human breast cancer cases, there was a strong inverse correlation between slug and ERalpha and E-cadherin immunoreactivity. Our findings indicate that ERalpha signaling through slug regulates E-cadherin and EMT.