HMGB1 is a member of the high-mobility group protein superfamily that has been widely studied as nuclear proteins that bind DNA, stabilize nucleosomes and facilitate gene transcription. A series of recent discoveries revealed a cytokine activity of HMGB1, that when secreted into the extracellular milieu, mediates downstream inflammatory responses in endotoxemia, arthritis and sepsis. HMGB1 is properly defined as a cytokine because it stimulates proinflammatory responses in monocytes/macrophages, is produced during inflammatory responses in vivo in standardized models of systemic and local inflammation, mediates delayed endotoxin lethality, and is required for the full expression of inflammation in animal models of endotoxemia, sepsis and arthritis. HMGB1 is either actively secreted by monocytes/macrophages or passively released from necrotic cells from any tissue. These pathways are central for the biology of HMGB1 as a cytokine, since they provide key mechanisms that integrate the inflammatory response to infectious and non-infectious cell injuries. Receptor signal transduction of HMGB1 occurs in part through the receptor for advanced glycation end-products (RAGE) expressed on monocytes/macrophages, endothelial cells, neurons and smooth-muscle cells. HMGB1 is a late-acting cytokine, because it first appears in the extracellular milieu 8-12 h after the initial macrophage response to proinflammatory stimuli. Knowledge of the cytokine role of HMGB1 has implications for understanding downstream cytokine cascades, regulation of delayed innate immune responses and targeting treatment towards these processes. Effectiveness of delayed treatment with HMGB1 blockade up to 24 h after induction of experimental sepsis offers a unique window of opportunities to allow rescue from lethal sepsis.