The hypothesis that bone marrow–derived, circulating endothelial cells incorporate into tumor blood vessels is unresolved. We have measured the numbers of bone marrow–derived versus resident endothelial cells in spontaneous prostate cancers during different stages of tumor progression and in age-matched normal prostates. Bone marrow–derived endothelial cells were rare in dysplasia and in well differentiated cancers representing between 0 and 0.04% of the total tumor mass. Instead, approximately 99% of all tumor-associated bone marrow–derived cells were CD45⁺ hematopoietic cells, including GR-1⁺, F4-80⁺, and CD11b⁺ myeloid cells. Similar to peripheral blood mononuclear cells, these tumor-associated myeloid cells expressed matrix metalloproteinases (MMPs), consistent with their proposed catalytic role during tumor angiogenesis. Furthermore, freshly isolated CD11b⁺ cells stimulated tumor endothelial cell cord formation by 10-fold in an in vitro angiogenesis assay. The bone marrow is, therefore, a reservoir for cells that augment tumor angiogenesis, but the tumor endothelium is derived primarily from the local environment.