Polypeptide growth factors, including members of the fibroblast growth factor (FGF) family, play an important role in the growth and maintenance of the normal prostate. We have found that FGF9 is expressed at high levels in the normal peripheral and transition zone of the human prostate. Analysis of FGF9 production by primary cultures of prostatic epithelial and stromal cells has shown that FGF9 is produced and secreted by the prostatic stromal cells. Neither of these processes appears to be modulated by androgens. Production of FGF9 by stromal cells in vivo was confirmed by immunohistochemistry. FGF9 is a potent mitogen for both prostatic epithelial and stromal cells in culture and is a more potent mitogen for these cells than either FGF2 or FGF7, two other FGFs expressed in the human prostate. FGF9 is an abundant secreted growth factor that can act as both a paracrine mitogen for epithelial cells and an autocrine mitogen for stromal cells. Western blot analysis of tissue extracts from the normal and hyperplastic transition zone shows that FGF9 is present at two to threefold higher levels in the hyperplastic transition zone. Overexpression of this paracrine and autocrine growth factor may play an important role in the epithelial and stromal proliferation in benign prostatic hyperplasia.