Radiographs of 175 patients with diffuse idiopathic skeletal hyperostosis (DISH) of the spine or ankylosing spondylitis were reviewed. DISH most frequently began in the middle and lower portions of the thoracic spine; it was rare in the upper portion. A few vertebrae were first affected, and then involvement extended, sometimes throughout the thoracic spine. The anterior and lateral aspects of vertebral bodies were mainly affected. Hyperostosis vertically spanning the anterior aspects of the vertebrae varied in thickness up to 10 mm, and the rate at which the hyperostosis proliferated was not specific for any vertebra. Males were 12 times more frequently affected than females, especially in the older age groups. Diabetes mellitus and hypertension have reportedly been associated with DISH, but no such correlation was found in this study. Despite the existence of criteria for differential diagnosis, it is sometimes difficult to distinguish ankylosing spondylitis from DISH radiologically. The radiologic features helpful in the differential diagnosis are described, and a review of the pertinent literature is included.