The role of cytokines during transplantation is currently an area of intense research. In particular, cytokines are increasingly appreciated as critical mediators of inflammatory responses to allogeneic tissue. Allogeneic bone marrow transplantation (BMT) involves cytokines not only in their capacity as hematopoietins but also as inducers of systemic inflammation after conditioning for BMT and during graft-versus- host disease (GVHD). GVHD is, in fact, a paradigm disease of systemic cytokine dysregulation, and the onset of severe acute GVHD can be considered a "cytokine storm." Advances in our understanding of the cytokine networks involved in BMT and GVHD should increase our ability to modify the complex interactions among cytokines and their cellular targets that lead to transplant-related morbidity.