Graft-versus-host reactions frequently produce cutaneous and systemic complications in patients receiving bone marrow transplants. Characteristic skin involvement typically heralds graft-versus-host reactions and significantly contributes to the morbidity associated with marrow transplants. Familiarity with these reactions and their treatment is important to dermatologists involved in the care of marrow transplant recipients. (J Am Acad Dermatol 1998;38:369-92.) Learning objective: At the conclusion of this learning activity, participants should be able to understand the requirements for development of graft-versus-host disease and the role of bone marrow transplantation and related risk factors. The immunopathophysiologic mechanisms of acute, chronic, and syngeneic graft-versus-host disease should be understood, and participants should be able to describe the clinical aspects of graft-versus-host reactions as well as treatment options for acute and chronic forms of the disease.