This study of chronic graft-versus-host disease (GVHD) describes the clinical, pathologic and laboratory features, and the causes of morbidity and mortality in 20 patients who received allogeneic marrow transplants from HLA identical sibling donors. Chronic GVHD is a pleiotrophic syndrome with variability in the time of onset, organ systems involved and rate of progression. The clinical-pathologic features resemble an overlap of several collagen vascular diseases with frequent involvement of the skin, liver, eyes, mouth, upper respiratory tract, esophagus and less frequent involvement of the serosal surfaces, lower gastrointestinal tract and skeletal muscles. Major causes of morbidity are scleroderma with contractures and ulceration, dry eyes and mouth, pulmonary insufficiency and wasting. Chronic GVHD has features of immune dysregulation with elevated levels of eosinophils, circulating autoantibodies, hypergammaglobulinemia and plasmacytosis of viscera and lymph nodes. In this study, three patients had limited chronic GVHD with relatively favorable prognosis characterized by localized skin involvement and/or hepatic disease without chronic aggressive histology. Most patients, however, had extensive disease with a progressive course. Survival was largely determined by the presence or absence of serious recurrent bacterial infections. The over-all severity of disease was best assessed by using the Karnofsky performance rating.