The association between histo-compatibility antigens and disease is reviewed, in particular that between HLA-B27 and spondylitic disorders, i.e., ankylosing spondylitis, Reiter's arthritis, psoriatic arthritis, and ankylosing hyperostosis. We determined whether the presence of HLA-B27 predicted specific radiographic findings and, conversely, whether specific radiographic changes predicted antigenic status. The prevalences of the HLA-B27 antigen in our patients were: ankylosing spondylitis, 100%; Reiter's arthritis, 93%; psoriatic arthritis, 55%; and ankylosing hyperostosis, 12%. The only specific radiographic finding associated with B27 positivity was severe spondylitis in psoriasis.