Background: Bone scintigraphy is a highly sensitive diagnostic nuclear medicine imaging technique that uses a radiotracer to evaluate the distribution of active bone formation in the skeleton related to malignant and benign diseases, as well as physiological processes. Subjective increase of the radioactivity at the sacroiliac joint in cancer patients could indicate physiological, metastatic, or inflammatory (sacroiliitis) cause. In this single-center study, we aimed to settle the normal sacroiliac index (SII) in males and females and if it differs from side to other, as a more accurate quantitative method than qualitative one, and assess its validity in discriminating the cause in pathological increase. Results: Sacroiliac index can differentiate physiological uptake at the SIJ from pathological uptake with a highly significant value (P < 0.001). Using SII at a cutoff value 1.17 had 100% sensitivity and specificity in differentiating between the two conditions. The normal SII in our institute ranged from 0.9 to 1.14 with no significant difference between right and left SIJs. The mean SII was 1.33 ± 0.11 in patients with ankylosing spondylitis (AS) and 1.38 ± 0.1 in metastatic cases with no significant difference in SII between the two diseased groups (P = 0.49). Conclusion: Bone scintigraphy is widely used in our institute and qualitative assessment of SIJ uptake is sometimes confusing. In this study, we assessed the normal SII range in physiological uptake and assessed a cutoff value (which is lower than other races) above which any increase in SIJ uptake is considered pathological. We could not confirm a cutoff value to discriminate between AS and metastatic affection of SIJ. Keywords: Ankylosing spondylitis, Metastasis, Scintigraphy, Sacroiliac index, Sacroiliac joint