Androgens are proposed to allocate finite energetic resources away from immune function and toward anabolic processes related to reproductive effort. In situations of pathogen exposure, the significant energetic demands associated with mounting an immune response are expected to produce a decrease in androgen levels and commensurate redistribution of energy. We tested the hypothesis that even the mild immune challenge associated with vaccination may cause a decline in men's testosterone. As predicted, men who received an influenza vaccination exhibited a more negative change in testosterone over a 2-week period than did men in a nonequivalent control group who were not vaccinated. These results suggest that men's androgen concentrations may be finely calibrated to trade-offs between the energetic demands of immune responses and other life history problems. Am. J. Hum. Biol., 2009. © 2008 Wiley-Liss, Inc.