Cytokine oncostatin M (OM) strongly inhibits the growth of MCF-7 breast cancer cells through the STAT3 signaling pathway. We have performed cDNA microarray analyses to identify novel OM-regulated genes in MCF-7 cells that are downstream effectors of the STAT3 signaling cascade. We show that expression of the calcium-binding protein S100A9 is strongly induced by OM in MCF-7 cells and this induction is markedly reduced in MCF-7-dnStat3 cells that express a dominant negative mutant of STAT3. We further show the induction of S100A9 by OM in other breast cancer cell lines whose proliferations are inhibited by OM whereas S100A9 is not significantly induced in SKBR-3 or HepG2 cells that do not respond to OM with a growth repression. In addition, inhibition of S100A9 expression with siRNA decreased cell response to OM-induced growth repression. By analyzing a series of S100A9 promoter reporter constructs, we have defined two discrete regions in the S100A9 promoter responsible for OM-induced transcriptional activation. Together these studies identify S100A9 as a novel OM-regulated gene through the STAT3-signaling cascade and suggest its involvement in the growth regulation of breast cancer cells.