Rheumatoid arthritis (RA) is a chronic immunological disease, the invasive monocytes/macrophages and lymphocytes present in synovial cells and synovial tissue produce many cytokines and inflammatory mediators by paracrine signaling and plays a role in the pathological progress in RA patients. Interleukin-18 (IL-18) is a representative proinflammatory factor and displays multiple biological functions. This study was designed to investigate the expression of IL-18 and its receptor (IL-18R) and IL-18 binding protein (IL-18BP) in serum, synovial fluid, and synovial tissue of patients with RA, and to identify the pathological role of IL-18 in RA. Serum, synovial fluid, and synovial tissue were obtained from RA patients. Samples from patients with osteoarthritis and healthy people were obtained as controls. Levels of IL-18, IL-18BP, and PGE2 in serum and synovial fluid were measured by enzyme-linked immunosorbent assay. The biological activity of IL-18 in serum and synovial fluid was detected on the basis of IFN-gamma secretion from IL-18-responding human myelomonocytic KG-1 cells. NO in serum and synovial fluid was detected by Griess reaction. Expression of IL-18, IL-18BP, IL-18R, iNOS, and COX-2 mRNA and protein in synovial tissues was determined by quantitative reverse transcriptase polymerase chain reaction and Western blot. This study shows the expression levels of IL-18, IL-18R, iNOS, COX-2, and the biological activity of IL-18 in both serum and synovial fluid and tissue of patients with RA were significantly increased compared with the corresponding samples from the two control groups. In addition, expression of IL-18BP in patients with RA was decreased compared with samples from the two control groups. In conclusion, the overexpression of IL-18 and IL-18R may play an important role in the pathogenesis of RA.