To evaluate the correlation between plain radiography and bone scintigraphy in the diagnosis of sacroiliitis, we analyzed 50 sacroiliac joints of 25 patients diagnosed as ankylosing spondylitis, The sacroiliac-sacral(SIS) ration significantly increases in patients with sacroiliitis of Grade l, ll, lll and was normal in completely ankylosed sacroiliac joints(Grade IV). However, there was no correlation between the degree of increase of SIS ratio and that of the radiographic changes in Grade l. ll. lll groups. Bone scintigraphy might be useful in the early detection of sacroiliitis. but plain radiography is more valuable in evaluating progression.