Acute graft-versus-host disease (GVHD), the major complication of allogeneic bone marrow transplantation (BMT), significantly limits the application of this important therapy. Advances in basic immunobiology, and particularly in the dissection of the complex networks of cytokines that affect the function of immune cells, have increased our understanding of the effector mechanisms of acute GVHD. Data from both experimental and clinical studies suggest that inflammatory cytokines are critical components of acute GVHD that can be modulated by a number of proteins, including anti-inflammatory cytokines and cytokine antagonists.