The IL-17 family consists of six related molecules, IL-17A, IL-17B, IL-17C, IL-17D, IL-17E, and IL-17F, of 163–202 aa that bear 20–50% homology to IL-17A, especially within the C-terminal region. IL-17C is 23% homologus with IL-17A. All IL-17 molecules share four conserved cysteine residues that may participate in the formation of intermolecular disulfide linkages. It has been indicated that IL-17 family members may induce inflammatory cytokines not only through activated T cells, but also through activated monocytes and macrophages. IL-17C binds IL-17RE which is selectively expressed in the lymphocyte compartment by Th17 cells; it has been indicated that IL-17C/IL-17RE could function in adaptive immunity to regulate T cell function. IL-17C stimulates epithelial inflammatory responses, including the expression of proinflammatory cytokines, chemokines and antimicrobial peptides, which are similar to those induced by IL-17A and IL-17F. However, IL-17C was produced by distinct cellular sources, such as epithelial cells, in contrast to IL-17A, which was produced mainly by leukocytes, especially those of the TH17 subset of helper T cells. IL-17 family members are involved in the pathogenesis of many inflammatory and autoimmune disorders, especially in the development of rheumatoid arthritis, other inflammatory arthritis disorders, and colitis. IL-17A, IL-17B, IL-17C, and IL-17F can affect inflammatory cytokine production of fibroblasts and macrophages. Recently, IL-17C expression in synovial fluid mononuclear cells and PBMCs of RA patients was reported.