To evaluat.e the diagnostic usefulness of quantitative sacroiliac joint scintigraphy and its clinica1 significance in the ankylosing spondylitis, we measured SIS ratio (Sacroiliac Joint/Sacrum Uptake ratio) by region of interest method in 52 patients with ankylosing spondylitis. As compared with radiological grade, SIS ratio was significantly elevated in the eariy stage of ankylosing spondylitis and returned to normal range in the late stage. Scintigraphic finding of focaI cptakes of radiotracer at multiple spinal apophyseal joints was charactnistic and proximal large joints were niore involved than distaI joints. The incidence and extent of the arthropathy revealed by scnitigraphy exceeded that which was clinically evident, and scintigraphy was more sensitive to detect syrnptomatic lesion than radiography. Therefore, it is suggested that bone scan is useful for the early diagnosis and the assessment of clinical activity of involved joints of ankylosing spondylitis.