Increasing numbers of persons are undergoing bone marrow transplantation for severe aplastic anemia and hematologic malignant neoplasms, and the proportion of those surviving has increased. Graft-v-host disease (GVHD) will, therefore, be seen with increasing frequency in the future. Graft-v-host disease occurs when immunocompetent donor lymphoid cells recognize and attack antigens of the immunocompromised host. We describe the clinical and histologic appearance of the different types of GVHD and discuss treatment, pathogenesis, and prevention of this entity.