Recent studies have demonstrated that the cellular contribution of the bone marrow to tumor neovascularization is highly complex. In this context, the extent to which bone marrow–derived cells incorporate as bona fide endothelial (nonhematopoietic) cells into perfused tumor vessels, or any new vessels formed postnatally (vasculogenesis), is unclear. To this end, we developed models to characterize local vessel–derived and bone marrow–derived endothelial cells (BMD-ECs). Then, we characterized the BMD-ECs based on a set of endothelial markers and morphology. Finally, we quantified their contribution to perfused blood vessels in tumors using transplanted as well as spontaneous primary and metastatic tumor models. We demonstrate that BMD-ECs incorporate in perfused tumor vessels, and that this contribution varies with organ site and mouse strain.