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Inflammation at the insertions of ligaments, tendons, or joint capsules to bone, which is termed enthesitis, is a characteristic feature of spondyloarthropathy. Because of the relative inaccessibility of the enthesis, the inflammatory, microbiologic, and immunologic events at that site have been poorly defined. Recent magnetic resonance imaging studies have drawn attention to the ubiquitous nature of enthesitis in spondyloarthropathies, especially adjacent to synovial joints. This may have implications for the mechanisms of synovitis in spondyloarthropathies. Magnetic resonance imaging studies also suggest that enthesitis lesions may be extensive, which could explain the diffuse nature of bone changes seen in some patients with spondyloarthropathies. The importance of enthesitis as a skeletal phenomenon in spondyloarthropathies has gained further support from transgenic models in which either tumor necrosis factor-a or bone morphogenetic protein-6 overexpression result in entheseal-associated polyarthropathy.