Collagen IX was identified as a distinct component of cartilage about 10 years ago. Composed of three polypeptide chains, its heterotrimeric molecules are located on the surface of type II collagen fibrils in cartilage. The interaction between collagens II and IX is stabilized by covalent crosslinks. The location of collagen IX molecules on fibril surfaces suggests that they represent macromolecular bridges between fibrils and other matrix components in cartilage, and that collagen IX is important for the cohesive and compressive properties of cartilage. Transgenic mice with mutations in a type IX gene develop normally, but show degenerative changes in articular cartilage after birth. Additional evidence for the importance of collagen IX in human articular cartilage comes from the recent finding that a mutation in one of the collagen IX genes causes multiple epiphyseal dysplasia. The mild complaints in affected individuals with the mutation suggest that mutations in type IX collagen genes may represent genetic risk factors for late onset osteoarthritis.