In this study, we investigated the differential expression profiles of cyclooxygenase-2 (COX-2) mRNA and proteins in osteoarthritis (OA) and rheumatoid arthritis (RA) patients to elucidate the role of COX-2 expression in the pathogenesis and development of these diseases and to provide novel drug targets for treating arthritis. A total of 60 patients who received arthroscopic surgeries for treating OA (N = 30) or RA (N = 30) were examined. Fifteen normal synovial tissue samples were included as the control group. Fibroblastic synovial cells in all samples were cultured in vitro and COX-2 mRNA, protein expression levels, and COX-2 levels were detected in synovial fluids by real-time polymerase chain reaction, western blotting, and enzyme-linked immunosorbent assay methods, respectively. The mRNA level of COX-2 was significantly elevated in synovial cells from OA and RA patients compared to that in control samples (P < 0.05). COX-2 mRNA level was significantly higher in synovial cells from OA patients than in those from RA patients (P < 0.05). Consistent results were obtained for COX-2 protein expression levels from patients' synovial samples. In synovial fluids, OA (P < 0.05), but not RA (P > 0.05), patients showed significantly higher COX-2 levels compared to the control group. Elevated synovial COX-2 expression facilitates the pathogenesis of OA and RA, and thus this index reflects the condition of these 2 diseases.