Bone scintigraphy is an extremely sensitive method for the detection of focal bone disease. In many hospitals, quantitative sacroiliac joint scintigraphy is still a routine procedure in detecting sacroiliitis. In previous studies, both 99Tcm-methylenediphosphonate (99Tcm-MDP) and 99Tcm-pyrophosphate have been used for bone imaging. 99Tcm-pyrophosphate is eliminated more slowly than 99Tcm-MDP from the circulation and gives a higher background activity. We wished to discover the sacroiliac/sacral ratio (SI/S ratio) changes when using different bone agents. The aim of this study was to evaluate differences in SI/S ratios between the two bone agents. Forty-six control subjects, aged 31-50 years, with no history of back pain, scoliosis, kyphosis, joint pain, arthritis, lesions within the pelvis, chemotherapy or systemic diseases such as diabetes or systemic lupus erythematosis, were included in the study. A posterior planar image of the pelvis was performed to calculate the SI/S ratio 3 h after the injection of 740 MBq 99Tcm-MDP or 99Tcm-pyrophosphate. Twenty-five subjects were studied with 99Tcm-MDP and 21 with 99Tcm-pyrophosphate. We found the SI/S ratios using 99Tcm-MDP to be slightly higher than those using 99Tcm-pyrophosphate, especially on the left side, but this difference was not statistically significant (P-values > 0.1 on both sides using Student's t-tests for unpaired data).