CD4⁺ T helper 1 (T_H1) and T_H2 cells have long been regarded as two sides of a coin in terms of adaptive immune responses. However, as I discuss here, this concept needs to be reconsidered. In particular, recent data indicate that interleukin-17 (IL-17) is produced by T_H cells that are distinct from the traditional T_H1- and T_H2-cell subsets. Furthermore, the generation of these IL-17-producing CD4⁺ T cells from naive precursors during immune responses is not dependent on the cytokines and transcription factors that mediate T_H1- and T_H2-cell development. Given that IL-17 has crucial roles in regulating tissue inflammation and the development of disease in several animal models of autoimmunity, I propose that IL-17-producing CD4⁺ T cells represent a distinct inflammatory T_H-cell lineage.