Keratinocyte growth factor (KGF) produces a rapid increase in the proliferation and motility of estrogen receptor (ER)-positive breast cancer cells which is abolished by estrogen deprivation and/or anti-estrogen treatment. The present study examined the hypothesis that ER-alpha is involved in the KGF proliferation in MCF-7 cancer cells using small interfering RNA (siRNA) to selectively inhibit ER-alpha expression.At 48 hours following ER-alpha siRNA transfection, the MCF-7 cells were treated with KGF (50 ng/ml) or vehicle for 24 hours. Cell proliferation was measured using a MTT assay. ER-alpha protein levels were quantified by Western blotting.ER-alpha siRNA transfection significantly reduced ER-alpha expression and MCF-7 cell proliferation. KGF-mediated enhancement of cell proliferation and motile cell morphology were reduced or absent in the siRNA transfected MCF-7 cells.ER-alpha expression is associated with KGF-induced proliferation of breast cancer cells.