Computed tomography (CT) was compared with plain radiography and quantitative sacroiliac (SI) scintigraphy in 28 patients with early ankylosing spondylitis (AS) of less than or equal to 10 years duration. Compared with conventional radiography, CT improved delineation of the SI joints and revealed more abnormalities and higher grades of sacroiliitis; this was significant in patients with early AS of less than or equal to 3 years duration. Quantitative sacroiliac scintigraphy showed higher SI joint: sacrum ratios of radioisotope uptake in patients with AS compared with controls. However, its diagnostic usefulness was limited by the frequency of inconsistent results and the lack of specificity. CT examination of the SI joints may be a useful adjunct in the diagnosis of early AS.