Now that ankylosing spondylitis is known to be relatively common, it is important that mild cases should be diagnosed earlier to avoid inappropriate investigation and unhelpful non-specific therapy and to improve social rehabilitation. However, difficulties in the diagnosis of ankylosing spondylitis are often encountered. To establish the clinical values of SPECT in ankylosing spondylitis, a study of radiologic evaluation, quantitative bone scan and SPECT have been carried out in 34 patients who were firmly suspected as having ankylosing spondylitis by other tests. This study showed that SPECT could improve the applicability of the imaging modality in the diagnosis and the assessment of disease activity and the capacity of early diagnosis in patients with ankylosing spondylitis. We believe that improved three dimensional localization in and around the sacroiliac joints by SPECT might overcome several diagnostic difficulties. Especially when radiographic abnormalities are not present in the early period, the diagnostic values of SPECT may be more potential.