One hundred and twenty-two hospitalized patients, 35 years or more of age with confirmed bilateral sacro-iliitis and 239 first-degree relatives, 25 years or more of age, were examined. All but 3 of the patients had confirmed ankylosing spondylitis (AS) in accordance with established criteria. The sacro-iliac joints were scored by a recently developed grading system. The prevalence of grade V sacro-iliits--extensive ankylosis--was greater in HLA B27-positive than in HLA B27-negative patients, and ankylosis of apophyseal joints, ossification of the interspinous ligament, or block vertebrae were not seen in HLA B27-negative patients. Shining corners and/or squared vertebrae were the most frequent findings of the dorsolumbar spine in patients with a disease course of less than 10 years. Mixed osteophytes were rare in patients below 40 years of age. All radiographic inflammatory changes scored in the spine-except for shining corners--were most often seen in patients with a disease history exceeding 20 years. Syndesmophytes and/or ankylosed apophyseal joints were less extensive in female than in male patients. No differences in the severity of radiographic changes in sacro-iliac joints or in frequency, severity or localization of different radiographic changes of the dorsolumbar spine were observed between HLA B27-positive patients with and those without psoriasis or acute anterior uveitis. Definite sacro-iliitis was restricted to HLA B27-positive relatives of HLA B27-positive probands and demonstrated in one-fifth of them. Shining corners and/or squared vertebrae were frequent findings in relatives with sacro-iliitis (45%), whereas apophyseal joint ankylosis, ossification of the interspinous ligament, block vertebrae and bridging syndesmophytes were not seen. Our results support the view that AS is not a defined disease entity and should be regarded as a syndrome. AS is an improper name for all patients fulfilling the Rome and/or New York criteria for definite AS, since many of them will probably never develop ankylosis of the spine or sacro-iliac joints.