Computed tomography (CT) was compared with plain radiography in 41 examinations of selected patients with a clinical history suggestive of sacroiliac joint disease. The obliquity of the sacroiliac joints renders radiographic interpretation difficult. In the 41 cases who were examined with standard anteroposterior and posteroanterior radiographs of the sacroiliac joints, four were normal, eight abnormal and 29 were equivocal. Equivocal findings included indistinct and possibly irregular articular margins to the joints and subarticular sclerosis. Of the 29 equivocal studies, nine were normal on CT and 20 were abnormal. CT demonstrated definite changes of sacroiliac joint disease in 29 of the 41 examinations, 16 of which were sacroiliitis and 13 osteoarthritis. With plain radiography four of the eight abnormal studies were consistent with sacroiliitis, and four with osteoarthritis. It is concluded that CT is more sensitive than plain radiography in the evaluation of sacroiliac joint disease, and is especially valuable when there are equivocal plain radiographs.