The morbidity and mortality associated with graft-host-disease (GVHD) is a significant obstacle to the greater use of allogeneic stem cell transplantation. Donor T cells that predominantly differentiate into TH1/Tc1 T cells and generate pro-inflammatory cytokines such as interferon-γ (IFN-γ) mediate GVHD. Although numerous studies have described a pathogenic role for IFN-γ, multiple reports have demonstrated that the lack of IFN-γ paradoxically exacerbated GVHD lethality. This has led to speculation that another subset of T cells may significantly contribute to GVHD mortality. Several groups have demonstrated a new lineage of CD4⁺ T helper cell development distinct from TH1 or TH2 differentiation. This lineage is characterized by production of interleukin (IL)–17A, IL-17F, IL-22, and IL-21 and has been termed TH17 cells. Here, we demonstrate that a highly purified population of TH17 cells is capable of inducing lethal GVHD, hallmarked by extensive pathologic cutaneous and pulmonary lesions. Upon transfer, these cells migrate to and expand in GVHD target organs and secondary lymphoid tissues. Finally, we demonstrate differential roles for tumor necrosis factor-α (TNF-α) and IL-17A in the clinical manifestations of GVHD induced by TH17 cells. Our studies demonstrate that cells other than TH1/Tc1 can mediate acute GVHD.