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VEGF-A, an angiogenic factor, is increased in the peritoneal fluid of women with endometriosis. The cytokine TGF-1 is thought to play a role in the establishment of endometriosis lesions. Inhibitor of DNA binding (ID) proteins are transcriptional targets of TGF-1 and ID1 has been implicated in VEGF-A regulation during tumor angiogenesis. Herein, we determined whether peritoneal expression of VEGF-A is regulated by TGF-1 through the ID1 pathway in women with endometriosis. VEGF-A was measured in peritoneal fluid by ELISA (n=16). VEGF-A and ID1 expression was examined in peritoneal biopsies (n=13), and primary peritoneal and immortalized mesothelial cells (MeT5A) by immunohistochemistry, qRT-PCR and ELISA. VEGF-A was increased in peritoneal fluid from women with endometriosis and levels correlated with TGF-1 concentrations (P<0.05). VEGF-A was immunolocalized to peritoneal mesothelium and TGF-1 increased VEGFA mRNA (P<0.05) and protein (P<0.05) in mesothelial cells. ID1 was increased in peritoneum from women with endometriosis and TGF-1 increased concentrations of ID1 mRNA (P<0.05) in mesothelial cells. VEGF-A regulation through ID1 was confirmed by siRNA in MeT5A cells (P<0.05). Our data supports role for ID1 in the pathophysiology of endometriosis, as an effector of TGF1 dependent upregulation of VEGF-A, and highlights a novel potential therapeutic target.