The ERa 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 ERa and E-cadherin expression by immunohistochemistry, suggesting that ERa 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 ERa signaling in ERa-transfected ERa-negative breast carcinoma cell lines, the MDA-MB-468 and the MDA-MB-231 and the effects of ERa knockdown in naturally expressing ERa-positive lines, MCF-7 and T47D. When ERa was overexpressed in the ERa-negative lines, 17b-estradiol (E2) decreased slug and increased E-cadherin. Clones maximally exhibiting these changes grew more in clumps and became less invasive in Matrigel. When ERa was knocked down in the ERapositive lines, slug increased, E-cadherin decreased, cells became spindly and exhibited increased Matrigel invasion. ERa signaling decreased slug expression by two different mechanisms: directly, by repression of slug transcription by the formation of a corepressor complex of ligand-activated ERa, 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-3b through phosphoinositide 3-kinase (PI3K)/protein kinase B (Akt). The GSK-3b inactivation, in turn, repressed slug expression and increased E-cadherin. In human breast cancer cases, there was a strong inverse correlation between slug and ERa and E-cadherin immunoreactivity. Our findings indicate that ERa signaling through slug regulates E-cadherin and EMT.