Abstract Thyroid cancer is the most common endocrine related cancer with the incidences occurring three to four times higher in women than in men, thus suggesting that estrogen plays a critical role in thyroid cancer progression. We have previously demonstrated that thyroid cancer cells express functionally active estrogen receptors (ER), ER-α and ER-α and that estrogen enhances the metastatic propensity of thyroid cancer cells by increased migration, adhesion and invasion, potentially mediated by enhanced secretion of matrix metalloproteinases (MMP), MMP-2 and MMP-9. However, to successfully grow and metastasize, thyroid cancer cells must initiate endothelial cell induced neo-vascularization such that adequate blood supply is maintained within the tumor. In an effort to define the interacting factors between endothelial cells and thyroid cancer epithelial cells, we developed an in vitro cell culture based system using three thyroid cancer cell lines (a papillary thyroid carcinoma cell line B-CPAP, and two follicular thyroid carcinoma cell lines, CGTH-W-1 and ML-1) and human umbilical vein endothelial cell (HUVECs). We observed that conditioned medium from estrogen treated thyroid cancer cells induced prolific tubulogenesis of endothelial cells which was abrogated in presence of the anti-estrogen, fulvestrant or neutralizing antibody to vascular endothelial growth factor (VEGF). We also observed that conditioned medium from thyroid cancer cells was able to induce migration of endothelial cells, again a phenomenon which was abrogated by fulvestrant and VEGF neutralizing antibodies. We further demonstrate that estrogen acts as a modulator of endothelial-epithelial cell interaction by modulating angiogenic factors that include VEGF and MMPs, which in turn induce mitogenic signaling and help promote and maintain cellular tumor heterogeneity. Estrogen modulated tumor modulating factors are viewed as novel therapeutic targets and provide evidence that the higher incidence of thyroid cancer in females is potentially attributed to the presence of a functional ER that mediates upregulation of pro-angiogenic factors and thus anti-estrogens can be potentially used to treat and/or prevent thyroid cancer. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1520. doi:1538-7445.AM2012-1520