Sacroiliitis of seronegative spondyloarthropathy may sometimes show on pelvis plain films findings indistinguishable from those of osteitis condensans ilii. Computed tomography (CT) can differentiate earlier than plain radiography between the 2 conditions; furthermore, it should also be possible to make this differentiation clinically. The aim of our study was to verify whether the criteria recently proposed by the European Spondylarthropathy Study Group (EESG) for the classification of spondyloarthropathy are useful. CT scans through the synovial part of the sacroiliac joints of 7 consecutive patients meeting the ESSG criteria and showing typical findings of osteitis condensans ilii on plain films were mixed with those of 15 consecutive patients with osteitis condensans ilii not meeting the ESSG criteria. Scans were examined for joint space and surface abnormalities blindly and independently by 2 observers. Six patients in the spondyloarthropathy group and one in the osteiitis condensans ilii group showed clear erosions and/or joint space narrowing of less than 2 mm in at least one joint. The difference was statistically significant (p less than 0.001). Our results suggest that by using criteria valid for the whole group of seronegative spondyloarthropathies, it is possible to differentiate clinically between seronegative spondyloarthropathies with sacroiliitis mimicking osteitis condensans ilii and "true" osteitis condensans ilii.