The purpose of this study was to analyze the effect of the soluble TNF-alpha receptor etanercept on the serum levels of IL-16, IL-17, IL-23, and macrophage inflammatory protein-3alpha (MIP-3alpha) in rheumatoid arthritis (RA) patients. Twenty-two patients with RA were administered etanercept once or twice a week for more than 6 months, and we evaluated clinical and laboratory parameters and serum levels of IL-16, IL-17, IL-23, and MIP-3alpha at the baseline and at 3 and 6 months. Additionally, the production of IL-23 and MIP-3alpha of cultured synovial cells stimulated with TNF-alpha from RA patients was determined by ELISA. We also used ELISA kits to determine synovial fluid (SF) levels of IL-17, IL-23, and MIP-3alpha in patients with RA, osteoarthritis (OA), pseudogouty arthritis (PGA), and gouty arthritis (GA). A significant decrease in serum levels of IL-23 and MIP-3alpha was observed at 3 and 6 months after initial treatment of etanercept. TNF-alpha induced MIP-3alpha but not IL-23 production in cultured synovial cells from RA patients. SF levels of IL-17, IL-23, and MIP-3alpha in RA patients showed significantly higher levels than those of OA, PGA, and GA patients. This study demonstrated that the reduction of IL-23 and MIP-3alpha production in RA patients was a newly determined function of etanercept.