Most knowledge about the synthesis of human tumor necrosis factor (TNF)-α and its regulation derives from in vitro studies. The hypothesis that the syndrome of malaise, myalgia, and low fever observed after a vaccination with live attenuated yellow fever virus could be associated with increased synthesis of TNF was investigated. Plasma samples of 15 healthy subjects taken before and until day 2 (until day 11 in 5 of these subjects) after yellow fever vaccination, showed a significant increase of plasma TNF concentration on day 2 after vaccination and a second peak on day 7. Interleukin-1 receptor antagonist (IL-1ra) concentration was significantly elevated in all subjects on day 2 after vaccination. In a control group receiving non-live vaccinations, TNF and IL-1ra concentrations were unchanged. Thus, yellow fever vaccination represents a routine medical intervention that results in a reproducible increase in the plasma concentration of TNF and secondary cytokines. This may serve as a unique model for the study of the regulation of TNF synthesis in vivo.