Marrow sinusoidal capillaries provide a niche for megakaryocyte progenitors and possibly hematopoietic stem cells. We sought to determine the fate of host sinusoidal capillaries during marrow transplant. We transplanted whole bone marrow, 2000 sorted Linneg Sca-1pos c-kitpos (SKL) hematopoietic stem and progenitor cells, or single SKL cells (along with rescue marrow) from male mice expressing green fluorescent protein into lethally irradiated female C57/BL6 hosts. We used green fluorescence and the presence of the Y-chromosome to identify donor-derived cells. Sinusoidal engraftment was studied from day 5 up to 1 year post-transplant and in secondary transplants. We identified numerous donor-derived cells based on green fluorescence or presence of the Y chromosome lining the sinusoids, but it was unclear whether these cells were vascular or hematopoietic. Furthermore, we were unable to demonstrate expression of von Willebrand factor, CD31 and MECA-32 using immunohistochemistry in healthy sinusoids. We used two methods to definitively identify donor-derived endothelial cells. First, we identified these cells based on location, characteristic shape and nuclei on H&E stained sections. Serial high power fields were photographed and then the same sections were stained with X and Y FISH probes. Second, we identified donor endothelial cells based on uptake of intravenously injected Ac-LDL which was endocytosed predominantly by sinusoidal endothelial cells four hours after injection. Flushed femoral bone marrow was treated with dispase and collagenase, and disaggregated cells were lineage depleted with a standard lineage cocktail containing Mac-1 antibody. Cytospins were photographed, Ac-LDL staining cells identified, and the same cytospins were stained with X and Y FISH probes. We counted numerous donor-derived endothelial cells whether 2X106 whole bone marrow (n=6 ), 2X104 SKL cells (n=6), or single SKL cells (n=2) were transplanted. These donor-derived endothelial cells were functional based on their ability to uptake DiI AcLDL. These cells appeared adjacent to mature, donor-derived megakaryocytes, suggesting function as a niche for megakaryocyte progenitors. Furthermore, donor-derived endothelial cells were present at levels similar to hematopoietic engraftment in every animal analyzed, suggesting robust levels of repair. Single cell transplants were assessed at seven and nine months, and in secondary transplants, establishing that these cells self-renew. To determine the mechanism whereby HSC’s repair the bone marrow sinusoids, we measured the expression of stromal derived factor-1 (SDF-1) at various time points during the first two weeks post-transplant by semi-quantitative RT-PCR, ELISA, and immunohistochemistry. SDF-1 expression peaked 3 days post-transplant and was expressed primarily by damaged blood vessels. The temporal and spatial pattern of engraftment matched expression patterns of SDF-1. This study demonstrates repair of host sinusoids by donor-derived endothelial cells following transplant, with levels of reconstitution similar to that of the hematopoietic system. This repair may be mediated by expression of SDF-1 by damaged marrow vasculature. Repair of sinusoidal capillaries may be a primary role of the HSC during transplant that is necessary for successful hematopoietic engraftment.