CD44 is a ubiquitous molecule also known as hyaluronic acid or homing receptor. However, its cellular functions and its role in inflammation, for example rheumatoid synovitis, are currently unknown. Here we propose a novel function for CD44. Using synovial cells from patients with rheumatoid arthritis (RA), we demonstrated that CD44 crosslinking and binding to hyaluronan augmented IL-6 production. Briefly, we found that (a) rheumatoid synovial cells highly expressed CD44; (b) crosslinking of CD44 augmented IL-6 production and its mRNA transcription; (c) hyaluronan, especially when fragmented, also increased IL-6 production; and (d) CD44 activated the transcription factor activator protein-1 and CAMP-responsive element binding protein. These results indicate that the adhesion of RA synovial cells to matrices such as hyaluronic acid through CD44 could activate transcription factor, resulting in cytokine production. We therefore propose that the function of adhesion molecules as a signaling molecule may be pivotal in the pathogenesis of inflammation, including RA synovitis.