Quantitative 99mTc pertechnetate sacro-iliac scanning (QTPS) was performed in 30 patients suspected of having sacro-iliac arthritis (SI). Abnormal scanning results but lack of definite radiographic changes were noted in all of them. Twenty-four of these patients were followed-up for an average of 3.4 years; roentgenological abnormalities were found in 23 patients, or 95%, namely definite SI in 13, slight abnormalities in 1 previously normal patient, inactive changes in 3, suspected SI was unchanged in the radiographs in 5 and regression of previously observed grad-I changes was found in 1. In only 1 patient was the radiological picture unchanged normal. No definite roentgenological SI developed in any of the 12 controls (patients with normal (QTPS) we followed in the same way. A close correlation was noted between the QTPS and the patients' complaints of low back pain/stiffness and the clinical findings, including the presence of HLA-B27. The present study confirms that, despite the absence of radiographic changes, QTPS allows identification of a group of clinical and laboratory features which together are sufficiently characteristic of SI to establish that diagnosis. QTPS also enables us to discover clinically silent SI.