Graft Versus Host Disease is caused by immunologically competent lymphocytes after the transfusion of blood products and bone marrow transplantation in the cases which are under chemoradiotherapy and in the patients who have congenital or acquired immunodeficiency, Disease has acute andchronicforms, it effects all the systems and charecterized by rash, bloody diarrhea, icterus and pancytopenia. In addition to supportive care, M-Prednisolon in acute form Cyclosporine A in the chronic form are effective drugs during the therapy of GVHD which has serious prognosis. Disease which has high mortality can be prevented by irradiating the blood and blood productis which will be infused. However it must be remembered that it would be an important complication that affects the success in bone marrow transplanted cases.