Advances in the molecular biology of human diseases indicate that the most striking manifestations of illness may be caused not by exogenous pathogenic or tumor products, but rather by toxic peptides produced by the host itself. Tumor necrosis factor (TNF), a polypeptide cytokine produced during infection, injury, or invasion, has proved pivotal in triggering the lethal effects of septic shock syndrome, cachexia, and other systemic manifestations of disease. Because removing TNF from the diseased host may prevent development of the illness, this factor has recently been the focus of intensive research. This review discusses the biology of this cytokine, with particular emphasis on its potential therapeutic role in septic shock and cachexia.