Interleukin (IL)-6 is a pleiotropic cytokine that has crucial roles in the regulation of immune response, inflammation and hematopoiesis. Recently, a new inflammatory helper T cell subset which produces IL-17A (IL-17), termed Th17 cells was identified and has been reported to be involved in the development and pathogenesis of collagen induced arthritis (CIA) and experimental autoimmune encephalomyelitis (EAE), which are known as the mouse model of rheumatoid arthritis and multiple sclerosis, respectively. It has been demonstrated that IL-6 together with TGF-beta induces the differentiation of Th17 cells from naive helper T cells in vitro. However, IL-6 independent pathway of Th17 differentiation has also been reported, suggesting that the role of IL-6 in vivo in these diseases remains unclear. With the treatment of anti-IL-6R antibody in CIA and EAE, we could suppress the differentiation of antigen specific Th17 cells and the onset of diseases. These results indicate that IL-6 is involved in the induction of Th17 cells in vivo, and anti-IL-6R antibody might be a promising therapy of Th17-mediated autoimmune diseases.