Interleukin (IL)-12 is a heterodimeric proinflammatory cytokine formed by a 35-kDa light chain (p35) and a 40-kDa heavy chain (p40). This cytokine is a key regulator of cell-mediated immunity, and therefore should have therapeutic potential in infectious diseases and tumors. Recently, a novel IL-12-associated cytokine, IL-23 has been discovered. IL-23 is also a heterodimer that consists of the p40 subunit of IL-12 and a novel subunit, p19. Several studies have shown that IL-23 possesses immunoadjuvant activity against tumor and infectious diseases as well as IL-12. On the other hand, there is increasing evidence that IL-12 and IL-23 have discrete roles in the regulation of T-cell-mediated immunity despite their structural similarities. IL-12 leads to the development ofinterferon-gamma-producing T-helper type 1 (Th1) cells, whereas IL-23 amplifies and stabilizes a new CD4(+) T-cell subset, Th17 producing IL-17. The IL-23/Th17 axis rather than the IL-12/Th1 axis contributes to several immune-mediated inflammatory autoimmune diseases. Furthermore, IL-23/IL-17 promotes tumor incidence and growth. Therefore, IL-23 and Th17 are attracting considerable attention at present. Taken together, these findings suggest that IL-23 may be an immunoadjuvant against infectious diseases and tumors, and a viable target for the treatment of inflammatory diseases.