GVHD, or graft-versus-host disease, is the leading cause of death following transplant surgery. Nearly 50% of all patients who develop GVHD will die as a result. GVHD occurs when the healthy cells of the donor tissue attack the immunocompromised cells of the recipient. The risk of GVHD is directly related to tissue compatibility and the biological relationship of the donor to the recipient, which is described in terms of HLA matching. Siblings and parents of the child that the stem cells came from will have the greatest chance of a perfect or close HLA match. The closer the HLA match, the lower your risk of GVHD.