The role of IL-17 and IL-17-producing CD4+ T cells in acute graft-versus-host disease (GVHD) has been controversial in recent mouse and human studies. We carried out studies in a murine acute GVHD model of fully major histocompatibility complex-mismatched myeloablative bone marrow transplantation. We showed that donor wild-type CD4+ T cells exacerbated acute GVHD compared with IL-17-/- CD4+ T cells, while IL-17 reduced the severity of acute GVHD. The augmentation of acute GVHD by transferred donor IL-17-producing CD4+ T cells was associated with increased Th1 responses, while IL-17 decreased the percentages of Th1 cells in the GVHD target organs. Furthermore, IL-17 reduced the infiltration of macrophages into the GVHD tissues. In vitro study showed that IL-17 could downregulate Th1 responses, possibly through inhibiting IL-12 production by donor macrophages. Depletion of macrophages in vivo diminished the protective effect of IL-17. Our results demonstrated the differential roles of adoptively transferred donor IL-17-producing CD4+ T cells and IL-17 in the same acute GVHD model.