To evaluate the diagnostic usefulness of quantitative sacroiliac joint scintigraphy and its clinical 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 early stage of ankylosing spondylitis and returned to normal range in the late stage. Scintigraphic finding of focal uptakes of radiotracer at multiple spinal apophyseal joints was characteristic and proximal large joints were more involved than distal joints. The incidence and extent of the arthropathy revealed by scnitigraphy exceeded that which was clinically evident, and scintigraphy was more sensitive to detect symptomatic 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.