Circulating and ex vivo production of interleukin (IL)-1beta, tumor necrosis factor (TNF)-alpha, IL-6, and IL-1 receptor antagonist (ra) and the diagnostic utility of these cytokines were studied in 123 patients with fever of unknown origin (FUO). Diagnoses were infections, 28; neoplasms, 14; noninfectious inflammatory diseases (NIID), 32; miscellaneous diseases, 10; and none made, 39. IL-1beta, IL-6, and IL-1ra concentrations were higher in patients with infections, neoplasms, and NIID than in healthy controls. Patients with infections had higher concentrations of TNF-alpha than controls. The ex vivo production of IL-1beta and IL-1ra in all patients with FUO did not differ from that in controls; however, production of TNF-alpha was lower in patients with neoplasms and NIID, and IL-6 production was lower in patients with neoplasms. Thirty-five patients with fever did not have elevated cytokines. Although some significant differences were found among the diagnostic subgroups, there was wide variation. Thus, measurement of these cytokines does not aid in the diagnosis of FUO.