Sacroiliac joints of 30 patients clinically diagnosed as ankylosing spondylitis were investigated by suprapubic sacroiliac radiography (SRR), sacroiliac computed tomography (SCT) and quantitative sacroiliac scintigraphy (QSS) and these methods were compared. In the case of diagnosing definite sacroiliitis, SSR was positive in 23 patients (76.6%), SCT in 25 patients (83.3%) and QSS in 17 patients (56.6%). Although there was no significant difference between SSR and SCT in the case of grading sacroiliitis, the number of normal patients was 7 with SSR and 3 with SCT. When grading was done by taking CT as standard, mean SI index values with QSS were in the normal range in grades 0, 1 and 4. In grades 2 and 3, SI index values showed sacroiliitis as compared with grade 4. Therefore, if plain radiography is normal or doubtful in patients clinically fitting ankylosing spondylitis, computed tomography must be the choice of investigation. On the other hand, QSS is effective in showing activation of inflammation quantitatively instead of diagnosing sacroiliitis. Thus it is useful in following physiopathological changes.