Microenvironmental cues instruct infiltrating tumor-associated myeloid cells to drive malignant progression. A subpopulation of tumor-associated myeloid cells coexpressing endothelial and myeloid markers, although rare in peripheral blood, are primarily associated with tumors where they enhance tumor growth and angiogenesis. These biphenotypic vascular leukocytes result from the endothelial differentiation of myeloid progenitors, a process regulated by tumor necrosis factor (TNF)α in vitro. An in vivo increase in tumor-derived TNFα expression promoted tumor growth and vascularity of mouse melanoma, lung cancer, and mammary tumors. Notably, tumor growth was accompanied by a significant increase in myeloid/endothelial biphenotypic populations. TNFα-associated tumor growth, vascularity, and generation of tumor vascular leukocytes in mouse melanoma tumors were dependent on intact host TNFα receptors. Importantly, TNFα-expressing tumors did not exhibit increased inflammation over control tumors, suggesting a unique action related to myeloid to endothelial differentiation. Our studies suggest that TNFα constitutes a tumor microenvironment signal that biases recruited monocytes toward a proangiogenic/provasculogenic myeloid/endothelial phenotype. [Cancer Res 2009;69(1):338–48]