Naïve T cells are multipotential precursors that differentiate into various effector subsets, such as T helper type 1 (Th1) and Th2 cells, which are characterized by their distinct functions. The IL-17-producing T helper (Th17) cell has been recently identified as a new subset of the T helper cell and a mediator of inflammation associated with various autoimmune diseases. Although several cytokines participate in Th17 cell development, IL-6 and TGF-β are key factors for the generation of Th17 cells from naïve T cells. On the other hand, IL-6 inhibits TGF-β-induced regulatory T (Treg) cells, which suppress adaptive T cell responses and prevent autoimmunity. Recent studies suggest that it is an effective approach in the treatment of various autoimmune and inflammatory diseases to normalize the balance between Treg and Th17 cell development. Here, we review the discovery of the Th17 subset, its properties and relationship with several autoimmune diseases.