A new method is described for quantitative measurement of the uptake of 99mTc--methylene diphosphonate (MDP) by the sacro-iliac joints. The method uses 'regions of interest' providing advantages over the previously described 'slice' method; the two methods are compared in normal subjects, patients with known sacro-iliitis and patients with low back pain. Sacro-iliac activity, as calculated by the sacro-iliac index (SII) in normal patients, was shown to decrease with age in females but not in males. The SII was compared with radiographs of the sacro-iliac joints in the patients with known sacro-iliac joint disease and in those with low back pain. The method is useful for the exclusion of sacro-iliitis as a specific cause of back pain.