Graft versus host disease (GVHD) occurs in 50% to 70% of patients receiving bone marrow transplants. It can also develop in immunosuppressed patients with malignancies who receive nonirradiated blood transfusions. Most work indicates that the primary mechanism of GVHD is cell-mediated. It is likely that humoral factors are involved as well. Cutaneous manifestations are the earliest and most frequent sign of the disease. These may be morbilliform, scarlatiniform, lichenoid, or sclerodermoid lesions. In acute GVHD, the skin, gastrointestinal tract, and liver are commonly affected. In chronic GVHD, findings similar to collagen vascular disorders are present. A skin biopsy establishes the diagnosis.