Select changes in microRNA (miRNA) expression correlate with estrogen receptor α (ERα) expression in breast tumors. miR-21 is higher in ERα positive than negative tumors, but no one has examined how estradiol (E ₂ ) regulates miR-21 in breast cancer cells. Here we report that E ₂ inhibits miR-21 expression in MCF-7 human breast cancer cells. The E ₂ -induced reduction in miR-21 was inhibited by 4-hydroxytamoxifen (4-OHT), ICI 182 780 (Faslodex), and siRNA ERα indicating that the suppression is ERα-mediated. ERα and ERβ agonists PPT and DPN inhibited and 4-OHT increased miR-21 expression. E ₂ increased luciferase activity from reporters containing the miR-21 recognition elements from the 3′-UTRs of miR-21 target genes, corroborating that E ₂ represses miR-21 expression resulting in a loss of target gene suppression. The E ₂ -mediated decrease in miR-21 correlated with increased protein expression of endogenous miR-21-targets Pdcd4, PTEN and Bcl-2. siRNA knockdown of ERα blocked the E ₂ -induced increase in Pdcd4, PTEN and Bcl-2. Transfection of MCF-7 cells with antisense (AS) to miR-21 mimicked the E ₂ -induced increase in Pdcd4, PTEN and Bcl-2. These results are the first to demonstrate that E ₂ represses the expression of an oncogenic miRNA, miR-21, by activating estrogen receptor in MCF-7 cells.