Leptin, a 16-kDa cytokine, has been implicated in several reproductive processes and disorders. Notably, elevated leptin levels in the peritoneal fluid of women with mild endometriosis has been demonstrated, suggesting a role for this cytokine in the early stages of disease establishment. To gain insight into the functional significance of leptin during the initial requisite proliferative and neovascularization events involved in endometriosis, we investigated the effect of disruption of in vivo leptin signaling on the establishment and/or maintenance of an endometriosis-like lesion in a syngeneic immunocompetent mouse model of endometriosis. Findings of this study show that the disruption of leptin signaling by ip injection of the pegylated leptin peptide receptor antagonist (LPrA) impairs the establishment of endometriosis-like lesions (derived from uteri of C57BL/6 female siblings) and results in a reduction of viable organized glandular epithelium, vascular endothelial growth factor-A expression, and mitotic activity. LPrA treatment resulted in a significant reduction of microvascular density in endometriosis-like lesions after continuous and acute courses. Endometriosis-like lesions (derived from tissue with functional leptin receptor) of Lepr(db) hosts (nonfunctional leptin receptor) were phenotypically similar to those of LPrA-treated mice. Our results confirm that leptin signaling is a necessary component in lesion proliferation, early vascular recruitment, and maintenance of neoangiogenesis in a murine model of endometriosis.