The S-I index (sacro-iliac/sacral uptakeX100) was calculated in 67 control subjects. We used both theposterior planar image and single photon emission computed tomography(SPECT) image for calculation in eachsubject. The S-I indixes were analysed according to age, gender and laterality. The mean right S-I index of 117.3, calculated by the posterior planar image, was signigicantly higher (p<0.01) than the left S-I index of 112.4. Themean right S-I index of 155.1, calculated by the SPECT image, was also significantly higher (p<0.05) than the leftS-I, index of 151.2. There was no significant difference of the S-I index according to gender by use of either theposterior planar image or SPECT image. There was tendency of the decrease of the S-I index with aging, which wassignificant only in the measurement by the posterior planar image. The correlation coefficient between both methods was 0.77 in the right S-I index, 0.79 in the left respectively. In regard to two different measurement, the age affected the S-I index less sisgnificantly in the SPECT image than in the posterior planar image. In termsof the superiority of one of these measurements the S-I index in the patients with sacroilitis must be comparedfor the access of discrepancy away from the control subjects before the SPECT can be used clinically.