Graft-versus-host disease (GVHD) is a major and often lethal complication of bone marrow transplantation. Research of the past few years has greatly expanded our understanding of the disease and enriched the arsenal of preventive and therapeutic procedures. The present review attempts to give a survey of experimental and clinical GVHD, updating essential knowledge with the latest information until July 1993. The covered topics include the complex immune pathomechanism of acute and chronic GVHD in murine models, the pathogenic role of major, minor, and other antigenic disparities, laboratory markers predicting GVHD, factors influencing appearance and course of the disease, the relationship between GVHD and the graft-versus-leukemia effect, and novel experimental and clinically tested preventive and therapeutic modalities. Finally, the authors set forth their perspective on the most relevant questions in GVHD-related research.