Fever is a phylogenetically ancient host response to infection, being found in fish and lizards, and conserved, with all its metabolic costs, in the higher mammals, including man. The conservation of the fever response in evolution is used as an argument for its survival value and, indeed, in experiments with cold-blooded animals "behavioral fever" has been demonstrated to reduce mortality associated with infection. Recent advances in the biology of interleukin-1 and other cytokines have allowed the testing, in vitro, of components of mammalian host defense (such as immune cell function) at temperatures typical of fever, and marked effects have been found. It remains to be demonstrated, however, that the hyperthermia of fever has survival value in man, and though it might be predicted that fever would be beneficial in infections, it is quite possible that in some circumstances even mild fever could be construed as harmful. In autoimmunity, for example, increased T-cell activation at febrile temperatures may well accelerate disease progress.