STAT4 is critical for the production of IFN-γ during the generation of Th1 immune responses. We investigated the role of STAT4 in mediating Th1-inducing activity of a vaccine adjuvant monophosphoryl lipid A (MPL-A) using the standard antigen ovalbumin (OVA) in STAT4KO mice. Our results show that splenocytes from STAT4KO mice displayed lower OVA-specific T-cell proliferation and IL-2 production compared with wild-type (WT) mice. Further, IFN-γ production was diminished in STAT4KO-derived splenocytes but the levels of IL-12 and TNF-α were similar compared with WT mice. Interestingly, STAT4 deficiency also led to a decrease in IL-10 and Th2 cytokines such as IL-4 and IL-13 upon MPL-A immunization, although IL-17 production was similar between WT- and STAT4KO-derived splenocytes. Our observations for defective Th1 and Th2 responses in STAT4KO mice were further supported by the low levels of Th1-associated IgG2a and Th2-associated IgG1 in the sera of these mice. Taken together, our results show that STAT4 plays a critical role in mediating both Th1 and Th2 responses upon immunization with MPL-A. Our study provides a better understanding of how MPL-A mediates T-cell activation which will be critical for future vaccine development.