Stem cell transplantation is often the only option for patients who have certain blood cancers. Unfortunately, this last resort can sometimes bring more harm than good. Graft versus host disease (GVHD), is a common complication that affects 50% of transplant patients. GVHD is not an issue if the patient's own stem cells are transplanted back. However, GVHD can become an issue, sometimes fatal, when stem cells from another donor are used.