Pituitary tumors are commonly encountered, and result from clonal expansion of a single mutated cell 1. Hypothalamic hormones, local growth factors and circulating sex steroid hormones promote pituitary tumor growth and expansion into large invasive tumors 2. Estrogen acting directly through its receptor and by stimulation of fibroblast growth factor regulates prolactin synthesis and secretion 3, 4. Fibroblast growth factor-2 (bFGF) modulates angiogenesis, tumor formation and progression in many tissues, including the anterior pituitary 5, 6, 7, 8. A pituitary tumor-derived transforming gene (PTTG) has been isolated, which is tumorigenic in vivo, regulates bFGF secretion, and inhibits chromatid separation 9, 10, 11. The human PTTG family consists of at least three homologous genes, of which PTTG1 is located on chromosome 5q33 12 and is expressed at low levels in most normal human tissues but is highly expressed in malignant human cell lines and in pituitary tumors 13. We report here that pituitary pttg is regulated in vivo and in vitro by estrogen. Maximal induction of rat pituitary pttg mRNA in vivo occurred early in pituitary transformation (normal cell to hypertrophic/hyperplastic cell), coincident with bFGF and vascular endothelial growth factor induction and pituitary angiogenesis. We also demonstrate that pttg expression is induced by bFGF, and show concordant pttg and bFGF expression in experimental and human pituitary adenomas. As bFGF and estrogen both induce pttg, and pttg expression coincides with the early lactotrophic hyperplastic response, angiogenesis and prolactinoma development, we propose a previously unknown paracrine growth factor-mediated mechanism for pituitary tumor pathogenesis and potentially other estrogen-regulated tumors.