Hyaluronan of high molecular weight is now widely used by intraarticular injection into knee joints in patients with osteoarthritis and rheumatoid arthritis. Proinflammatory cytokines and matrix degradation products (matrikines) increased in osteoarthritic and rheumatoid arthritic joints induce catabolic enzymes such as collagenase and aggrecanase that cause cartilage degradation. The recent studies have shown that hyaluronan inhibits the catabolic actions by cytokines and matrikines via its cell surface receptors. This paper describes the chondroprotective effects of hyaluronan on osteoarthritis and rheumatoid arthritis.