The similarities between diffuse idiopathic skeletal hyperostosis (DISH) and some forms of ankylosing spondylitis suggest shared pathogenic mechanisms. Entheseal ossification progresses at the same rate in the two conditions, and spondyloarthritis was the first diagnosis considered in several families with genetically determined early-onset DISH. However, DISH may be a heterogeneous condition, as the presence of peripheral calcifications in some families suggests pathogenic similarities with several animal models combining entheseal ossification and peripheral calcifications, as well as with X-linked familial hypophosphatemia and dentin-matrix-protein mutations. In the far more common presentation of hyperostosis without calcifications, entheseal ossification may be related to abnormal osteoblastic differentiation of mesenchymatous stem cells normally found around the intervertebral disks, in the vertebral periosteum, and in the anterior and posterior longitudinal ligaments. The many factors suspected of promoting this abnormal differentiation include bone morphogenetic proteins (BMPs), retinoids, and various hormonal factors; in addition, adipokines such as leptin are the focus of growing interest based on the well-documented association between DISH and obesity. Confirmation of the role for mesenchymatous cells in DISH should encourage investigations of mesenchymatous cells as possible pathogenic contributors to the entheseal abnormalities seen in spondyloarthritis. These cells normally exert immunosuppressive effects, which may be subverted in spondyloarthritis, notably by a T-cell population that homes specifically to the entheses.