The failure to engraft occurs in approximately 5% of HSC transplant recipients. Primary graft failure occurs when a patient's absolute neutrophil count is below 500 neutrophils/µL on day 28 post-transplant. Secondary graft failure occurs when the patient develops pancytopenia. Graft failure may result from transplantation of inadequate numbers of HSCs, infection, immunosuppressive drug toxicity, or graft rejection due to HLA differences. As grafts are rejected, peripheral blood counts decrease, and chimerism studies will show the loss of donor cells in the bone marrow and peripheral blood. During this period patients must be protected from infection due to neutropenia. The most common treatment for graft failure is a “salvage” transplant with cells from the original donor or an HLA compatible donor.