Present diagnostic criteria for ankylosing spondylitis (AS) lean heavily on the x-ray examination, but there is much dispute as to its efficacy, especially in mild or early cases. Determinations of the HLA B27 histocompatibility antigen appear to define the population at risk far better than any other means. Of 31 patients who had the HLA B27 antigen, all had negative latex fixation tests and axial polyarthritic complaints (seronegative spondyloarthropathy or rheumatoid variant). Three had Reiter's syndrome and one had ulcerative colitis. Of the remaining 27 patients, nine had definite AS, 11 had probable AS, and seven had possible AS. Eleven of the 27 underwent at least one invasive spinal procedure (myelogram, laminectomy, fusion, facet denervation) before a diagnosis of AS was made.