The host responds to infectious, toxic inflammatory and immunological challenges with a remarkably consistent set of changes which are often grouped together and called the acute phase response. This response includes fever, negative nitrogen balance, increased synthesis of certain hepatic proteins, decreases in plasma iron and zinc levels and increases in circulating neutrophils. The continued presence of the disease results in the persistence of acute phase changes such that these changes can also be markers of a chronic pathological process. The ability of the host to produce several components of the acute phase response is due to the synthesis and release of a mediator peptide derived from activated mononuclear phagocytes and now called interleukin-1 (IL-1). Many of the clinical and laboratory changes noted in chronic hemodialysis are indicative of an acute phase response and are probably mediated by IL-1. This review considers the production and biological properties of IL-1 and the role of this mediator in host responses. In addition, particular attention is paid to the importance of monocyte activation and IL-1 release which may be a consequence of hemodialysis.