Heterotopic ossification following hip surgery occurred in three patients with ankylosing hyperostosis of the spine. No technical difficulty during surgery was encountered in these individuals. The occurrence of this postoperative complication, coupled with the appearance of bony outgrowths at sites of ligament attachment throughout the axial and extra-axial skeleton in patients with ankylosing hyperostosis of the spine, suggests the presence of an underlying ossifying diathesis, diffuse idiopathic skeletal hyperostosis (DISH). A significant number of patients with DISH possess the second segregant series antigen, HLA-B27, a feature they share with individuals with other arthropathies characterized by abundant ossification; this gene may be closely related to one which influences bone formation. The possible association of postoperative heterotopic ossification and ankylosing hyperostosis of the spine indicates that a radiographic examination of the vertebral column in patients undergoing hip surgery may be a useful screening procedure.