Diffuse idiopathic skeletal hyperostosis (DISH) and ankylosing spondylitis (AS) share involvement of the axial skeleton and peripheral entheses. Both diseases produce bone proliferations in the later phases of their course. Although the aspect of these bone proliferations is dissimilar, confusion of radiologic differential diagnosis between the two diseases exists mostly as a consequence of a lack of awareness of their characteristic clinical and radiographic features. The confusion may extend to the clinical field because both advanced DISH and advanced AS may cause the same limitations of spinal mobility and postural abnormalities. However, the radiologic spinal findings are so different that changes due to each disease can be recognized even in patients in whom both diseases occur. This article reviews the clinical and radiologic characteristics that should help clinicians differentiate between the two diseases without much difficulty.