Technical Abstract: Stem and progenitor cells derived from adult marrow have been shown to regenerate vascular cells in response to injury. However, it is unclear whether the type of injury dictates the contribution of such cells to neovascularization and which subpopulations of cells contribute to vascular regeneration. To address these questions, we determined the extent that hematopoietic stem cells (HSC) contributed to blood vessel formation in response to two types of liver injury: partial hepatectomy (PH) and toxin-induced injury. Lac-Z-labeled HSC were engrafted into lethally irradiated, genetically matched recipients. After 14 d, we identified transplanted cells engrafted within the vascular endothelium of toxin-damaged liver, but not in the vasculature of liver regenerated in response to PH. Engraftment of HSC-derived cells occurred in a gradient fashion, with the highest activity in the severely injured areas. Although HSC-derived cells contributed to both microvessels and large vessels, the large caliber vessels trended toward higher engraftment levels. Thus, the contribution of marrow-derived cells to hepatic neovascularization is dependent upon the type of injury sustained. Furthermore, following toxin-induced liver injury, engraftment rates trended higher in large vessels compared with capillaries, suggesting that remodeling of existing vessels is a predominant mechanism of repair, relative to the formation of new microvasculature.