In breast tumors the expression of estrogen receptor alpha (ERα) is known to be associated with a more favorable prognosis. ERα expression has been reported to reduce the metastatic potential of breast cancer cells. Recently, we have observed that extracellular matrix proteins activate ERα and that both liganded and unliganded receptor modulate cell invasiveness acting at nuclear level. To explain the mechanisms by which ERα regulates cell adhesion, we have evaluated the expression of α(5)β(1) integrin, prevalently expressed in stationary cells, in response to 17β-estradiol (E2). Here we show that E2/ERα increases the expression of integrin α(5)β(1) through Sp1-mediated binding to a GC-rich region located upstream of an ERE half-site in the 5' flanking region of the α(5) gene forming a ternary ERα-Sp1-DNA complex. Estrogen responsiveness of the α(5) gene promoter, as observed in HeLa cells, underlies a general mechanism of regulation which is not strictly linked to the cell type. Our data reveal novel insight into the molecular mechanisms sustaining the reduced invasiveness of ERα expressing cells demonstrating that α(5)β(1) integrin expression is related to the maintenance of the stationary status of the cells, counteracting E2/ERα capability to enhance breast cancer cell migration and invasion.