Several lines of evidence indicate that Flt-1, a fms-like tyrosine kinase receptor, which binds to vascular endothelial growth factor (VEGF)-A, VEGF-B, and PlGF, is a positive regulator of angiogenesis in the context of tumor growth and metastasis. However, the molecular basis of its action is still not clear. Besides endothelial cells, Flt-1 is also expressed by other different cell types, including myeloid hematopoeitic cells (monocytes and macrophages). To examine the functions of Flt-1 expressed by bone marrow–derived myeloid cells in supporting tumor growth and angiogenesis, Flt-1 tyrosine kinase–deficient (Flt-1 TK−/−) bone marrow cells were transplanted into lethally irradiated syngeneic recipients. After hematopoietic reconstitution, we orthotopically implanted syngeneic wild-type glioma cells or glioma cells overexpressing either VEGF₁₆₄ or PlGF-2. Loss of Flt-1 signaling in bone marrow–derived myeloid cells led to a significant decrease in tumor volume and vascularization in gliomas. VEGF but not PlGF overexpressed by glioma cells restored the tumor growth rate in Flt-1 TK−/− bone marrow chimera. VEGF and PlGF overexpression by tumor cells induced an accumulation of bone marrow–derived myeloid cells into tumor tissue. This infiltration was decreased in tumors grown in Flt-1 TK−/− bone marrow chimeras. When investigating chemokines and growth factors involved in myeloid cell recruitment, we determined elevated SDF-1/CXCL12 levels in VEGF- and PlGF-overexpressing tumors. Collectively, these results suggest that Flt-1 signaling in myeloid cells is essential to amplify the angiogenic response and to promote glioma growth. [Cancer Res 2008;68(18):7342–51]