The radiological confirmation of sacro-iliitis is essential for the early diagnosis of ankylosing spondylitis. Conventional radiography is too insensitive to detect early changes in these joints, and radionuclide scintigraphy is a non-specific, though highly sensitive, technique. This study describes a preliminary survey of the use of computed tomography (CT) of the sacro-iliac joints in the diagnosis of early sacro-iliitis. Patients were selected for study from a routine rheumatology clinic. Entry criteria included a clinical suspicion of sacro-iliitis and normal or equivocal findings on conventional radiography. Twenty-two patients were selected for study. Nine had normal plain films and normal CT; four had equivocal plain films with conclusive evidence of sacro-iliitis on CT; in two cases, conventional radiographs were normal but CT showed clear evidence of sacro-iliitis: two other patients had equivocal findings on straight X-ray examination but normal CT. Computerized tomography of the sacro-iliac joints can be useful in the early diagnosis of sacro-iliitis if conventional radiography is equivocal or normal. Further controlled studies are necessary to establish the sensitivity and specificity of the technique.