High mobility group box 1 (HMGB1) is a highly conserved, ubiquitous protein present in the nuclei and cytoplasm of nearly all cell types. We recently discovered that HMGB1 is secreted into the extracellular milieu and acts as a proinflammatory cytokine. Administration of HMGB1 to normal animals causes inflammatory responses, including fever, weight loss and anorexia, acute lung injury, epithelial barrier dysfunction, arthritis, and death. Anti-HMGB1 treatment, with antibodies or specific antagonists, rescues mice from lethal endotoxemia or sepsis and ameliorates the severity of collagen-induced arthritis and endotoxin-induced lung injury. Here, we give an abridged review of the cytokine activity of HMGB1, its secretion and release into the extracellular milieu, the putative signal transduction pathways, including interaction with cell-surface receptors and intracellular signaling, and its role in several inflammatory diseases. Finally, the therapeutic potential of blocking HMGB1 in the treatment of inflammatory diseases is discussed.