Sacroiliitis is often difficult to diagnose with certainty using conventional radiographs and radionuclide scanning. Computed tomography was used to study the sacroiliac joints in 20 consecutive patients clinically suspected of having sacroiliitis on initial evaluation. Of these 20 patients, 17 fulfilled conventional clinical criteria for sacroiliitis; computed tomography demonstrated changes of sacroiliitis in 12 of these 17 patients. CT revealed no changes of sacroiliitis in the three patients who failed to meet conventional clinical criteria for sacroiliitis. Conventional radiography demonstrated diagnostic changes of sacroiliitis in only five patients. Seven patients with CT changes of sacroiliitis had equivocal or negative conventional films. CT therefore seems superior to conventional radiography in detecting sacroiliitis.