Due to its unique bicompartmental anatomy and spatial configuration, the sacroiliac joint can be more accurately defined by computed tomography (CT) than conventional radiography. Using a tilted gantry and paraaxial scanning technique, the synovial portion of the joint is oriented vertically on the CT image, while the ligamentous portion is oriented oblique-horizontally. The tilted CT gantry technique allows full ventral-dorsal imaging of the synovial portion of the sacroiliac joint. We have found the accuracy of CT to be superior to conventional radiography in the detection of early erosive sa- croiliitis and joint space narrowing. In all patients with discrepancy between the two radiologic techniques, the changes were either only demonstrated or better demonstrated by CT than conventional radiography. © 1982 Raven Press, New York.