The S-I indes (sacro-iliac/sacral uptake$ imes$100) was calculated in 67 control subjects. We used both the posterior planar image and single photon emission computed tomography (SPECT) image for calculation in each subject. The S-I indices were analysed according to age, gender and laterality. The mean right S-I index of 117.3, calculated by the posterior planar image, was significantly higher (p<0.01) than the left S-Iindex of 112.4 The mean right S-I index of 155.1, calculated by the SPECT image, was also significantly higher (P<0.05) than the left S-Iindex according t6o gender by use of either the posterior planar image or SPECT image. There was a tendency of the decrease of the S-I index with aging, which was significant 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 signifi antly in the SPECT image than in the posterior planar image. In terms of the superiority of one of these measurements the S-Iindex in the patients with sacroiliitis must be compared for the access of discrepancy way from the control subjects before the SPECT can be used clinically.