The epithelium-specific Ets transcription factor, PDEF, plays a role in prostate and breast cancer, although its precise function has not been established. In prostate cancer, PDEF is involved in regulating prostate-specific antigen expression via interaction with the androgen receptor and NKX3.1, and down-regulation of PDEF by antiproliferative agents has been associated with reduced PDEF expression. We now report that reduced expression of PDEF leads to a morphologic change, increased migration and invasiveness in prostate cancer cells, reminiscent of transforming growth factor β (TGFβ) function and epithelial-to-mesenchymal transition. Indeed, inhibition of PDEF expression triggers a transcriptional program of genes involved in the TGFβ pathway, migration, invasion, adhesion, and epithelial dedifferentiation. Our results establish PDEF as a critical regulator of genes involved in cell motility, invasion, and adhesion of prostate cancer cells. [Cancer Res 2007;67(9):4219–26]