Fibroblast growth factor-9 (FGF-9) is a steroid-regulated mitogen and survival factor for nerve and mesenchymal cells. In the current study, we determined the expression pattern and functional roles of FGF-9 in the ectopic endometriotic lesions. We found that FGF-9 and its receptors were effectively expressed by ectopic endometriotic tissues. The expression of FGF-9 was greater in the early stage of endometriosis, compared with the severe stage, which is consistent with concentration of 17 beta-estradiol in the peritoneal fluid of women with endometriosis. In addition, expression of FGF-9 in ectopic endometriotic stromal cell was inhibited by treatment with ICI 182,870 indicating it is likely regulated by estrogen in an autocrine manner. Administration of 17 beta-estradiol induced FGF-9, FGF receptor 2IIIc, and FGF receptor 3IIIc expression in endometriotic stromal cells. Concordant with this result, treatment of endometriotic stromal cells with 4-hydroxyandrostenedione (an aromatase inhibitor) or ICI 182,870 inhibited their proliferation, and that was reversed by coadministration with 17 beta-estradiol or FGF-9. In conclusion, expression of FGF-9 in endometriotic stromal cells is associated with aberrant production of estrogen. The capability of proliferation possessed by endometriotic stromal cell during menstruation when ovarian 17 beta-estradiol is in the nadir may be mediated, at least in part, by autocrined estrogen-stimulated expression of FGF-9 and its receptors.