Soluble tumour necrosis factor receptors (sTNF-Rs) play a role as modulators of the biological function of tumour necrosis factor-alpha (TNF-alpha) in an agonist/antagonist pattern. In various pathologic states the production and release of sTNF-Rs may mediate host response and determine the course and outcome of disease by interacting with TNF-alpha and competing with cell surface receptors. The determination of sTNF-Rs in body fluids such as plasma or serum is a new tool to gain information about immune processes and provides valuable insight into a variety of pathological conditions. Regarding its immediate clinical use, sTNF-Rs levels show high accuracy in the follow-up and prognosis of various diseases. In HIV infection and sepsis, sTNF-Rs concentrations strongly correlate with the clinical stage and the progression of disease and can be of predictive value. Determination of sTNF-Rs also gives useful information for monitoring cancer and autoimmune diseases. The information provided is often even superior to that obtained with classical disease markers, probably due to the direct involvement of the "TNF system" in the pathogenetic mechanisms in these patients. The available data imply that the measurement of sTNF-Rs, especially of the sTNF-R 75kD type, is a useful adjunct for quantification of the Th1-type immune response, similar to other immune activation markers such as neopterin and beta 2-microglobulin. Endogenous sTNF-Rs concentrations appear to reflect the activation state of the TNF-alpha/TNF receptor system.