To evaluate clinical usefulness of quantitative sacroiliac scintigraphy (QSS) in detecting sacroiliitis, we used a modified, pixel by pixel technique for calculating sacroiliac joint/sacrum uptake ratios (sacroiliac joint index - SII). We studied 90 controls, 18 selected patients with active sacroiliitis, 2 ankylosing spondylitis patients with completely ankylosed sacroiliac joints, 14 patients with nonspecific low back pain and 5 patients with rheumatoid arthritis. In the controls, we found that the SII decreases with increasing age (P less than 0.001) and is higher in males than in females (P less than 0.005). In the patients with active sacroiliitis, 9 out of 14 older than 30 had an abnormal SII; 3 of these patients showed no radiographic or CT abnormalities of the sacroiliac joints. None of the 4 patients with sacroiliitis under 30 years of age had values which fell out of the normal range for their age and sex. Only 1 of the 14 patients with non-inflammatory low back pain had an abnormally high SII. A borderline SII was found in 1 of the 5 patients with rheumatoid arthritis. QSS may be useful in detecting active sacroiliitis, sometimes even before the occurrence of radiologic abnormalities. However, because of its low sensitivity, its clinical usefulness is limited, especially in patients under 30 years of age.