Interleukin-6 (IL-6) is a multifunctional cytokine that presumably plays its major role as a mediator of several of the acute phase inflammatory responses. These include inflammatory cell and lymphocyte activation and hepatocellular stimulation of acute phase protein synthesis. IL-6 expression is normally low, and serum levels are usually non-detectable in the absence of inflammation. However, with advancing age, serum levels become detectable, and it is proposed that this reflects an age-associated loss in the normal regulation of gene expression for this molecule. The cause of this is most likely multi-factorial, but there is evidence that it relates to an age-associated loss of T cell immunoregulatory functions as well as menopausal loss of estrogen. In any event, the "inappropriate" presence of IL-6 results in many changes typical of chronic inflammation. There is also speculation that IL-6 may contribute to the pathogenesis of several diseases of late-life including lymphoma, osteoporosis, and Alzheimer's disease. In this review the biology of this important cytokine is presented and its relevance to gerontology is highlighted.