Over the period 1976-83 the clinical records were studied of 150 patients with the diagnosis of confirmed or possible sacro-iliitis. All these patients had pathologic sacro-iliac indices on quantitative bone scintigraphy. The most recent radiographs of the sacro-iliac joints were examined by two radiologists independently of each other. 103 (68%) patients with a mean duration of symptoms of 6.7 years, had normal radiographs. The frequency of HLA B27 in this group was only 27%, contrary to the expected 90-100% in an ankylosing spondylitis (AS) population. In a follow-up study, 30 patients, who in 1976-79 had normal sacro-iliac joint X-rays, were further investigated. The mean duration of symptoms was 11.3 years. 21 patients (70%) still had normal radiographs of their sacro-iliac joints and the HLA B27 frequency in this group was 28%. Normal radiographs of the sacro-iliac joints, in spite of the long duration of symptoms together with a low frequency of HLA B27, makes the diagnosis of AS most unlikely. This indicates a low specificity for quantitative sacro-iliac scintigraphy in the early diagnosis of AS. In the follow-up study, Calin's screening test for AS was included and was also found to have a low specificity.