To measure the levels of interleukin-34 (IL-34) in serum and synovial fluid (SF) of patients with rheumatoid arthritis (RA) and to evaluate the effect of recombination human (rh) IL-34 on IL-17 production by peripheral blood mononuclear cells (PBMC) in RA patients, the serum and SF levels of IL-34, and the production of IL-17 by rhIL-34-treated PBMC of RA patients were measured by enzyme-linked immunosorbent assay. We also tested the change of IL-34 level after tumor necrosis factor (TNF)-α blockade therapy in 30 RA patients. In contrast to almost no detectable IL-34 in osteoarthritis (OA) and healthy serum, IL-34 could be detected in 93 out of the 125 RA cases (74.4%). Sera IL-34 levels were significantly higher in RA patients compared with the controls and correlated with disease activity. IL-34 levels were higher in SF samples than in sera in 11 RA patients. The level of serum IL-34 decreased after anti-TNF treatment. In the presence of rhIL-34, stimulation of PBMC from RA patients resulted in increased production of IL-17. These findings suggest that IL-34 may play a role in the pathogenesis of RA.