IL-17-secreting CD8 T cells (Tc17) have been described in several settings, but little is known regarding their functional characteristics. While Tc1 cells produced IFN-gamma and efficiently killed targets, Tc17 cells lacked lytic function in vitro. Interestingly, the small numbers of IFN-gamma-positive or IL-17/IFN-gamma-double-positive cells generated under Tc17 conditions also lacked lytic activity and expressed a similar pattern of cell surface proteins to IL-17-producing cells. As is the case for Th17 (CD4) cells, STAT3 is important for Tc17 polarization, both in vitro and in vivo. Adoptive transfer of highly purified, Ag-specific IL-17-secreting Tc17 cells into Ag-bearing hosts resulted in near complete conversion to an IFN-gamma-secreting phenotype and substantial pulmonary pathology, demonstrating functional plasticity. Tc17 also accumulated to a greater extent than did Tc1 cells, suggesting that adoptive transfer of CD8 T cells cultured in Tc17 conditions may have therapeutic potential for diseases in which IFN-gamma-producing cells are desired.