CD4⁺ effector T cells have been categorized into two subsets: T helper type 1 (T_H1) and T_H2. Another subset of T cells that produce interleukin 17 (IL-17; 'T_H-17 cells') has been identified that is highly proinflammatory and induces severe autoimmunity. Whereas IL-23 serves to expand previously differentiated T_H-17 cell populations, IL-6 and transforming growth factor-β (TGF-β) induce the differentiation of T_H-17 cells from naive precursors. These data suggest a dichotomy between CD4⁺ regulatory T cells positive for the transcription factor Foxp3 and T_H-17 cells: TGF-β induces Foxp3 and generates induced regulatory T cells, whereas IL-6 inhibits TGF-β-driven Foxp3 expression and together with TGF-β induces T_H-17 cells. Emerging data regarding T_H-17 cells suggest a very important function for this T cell subset in immunity and disease.