Diffuse idiopathic skeletal hyperostosis is characterized by new bone growth at the point of insertion of ligaments and tendons to bone. We examined retrospectively the anatomical morphologic changes discernible at the insertion of spinal longitudinal ligamentous fibrous tissue to vertebral bodies. The earliest evidence of bone formation was in the "waist" of the vertebral body away from the intervertebral disc area. New bone arose along the insertion of the fibrous tissue to the anterior cortical surface of the vertebral body and progressed along the fibres at an angle to the cortical surface distinct from it until the advanced stages. With disc degeneration the 2 processes were distinct and separate. Degenerative disc disease occurred at the margin of the endplate of the vertebral body with associated changes in the disc itself. Entheseal ossification occurred remote from the margin of the intervertebral disc and remained distinct from the subjacent vertebral body as it followed the ligamentous tissue; fusion with the cortical surface of the subjacent vertebral body was only seen in the most advanced cases of disseminated idiopathic systemic hyperostosis.