1. The levels of basic fibroblast growth factor (FGF) expression and secretion and its messenger ribonucleic acid (mRNA) expression in well-differentiated endometrial cancer (Ishikawa) cells were significantly increased by estradiol. 2. This increase was significantly inhibited by tamoxifen, progestins (progesterone, medroxyprogesterone acetate [MPA], and 17 alpha-hydroxyprogesterone), and to some extent danazol, but not by terahydrocortisol and hydrocortisone. 3. Estrogen might stimulate the basic FGF secretion of endometrial cancer cells, at least for neovascularization, and antiestrogenic compounds may inhibit the estrogen-induced event.