WHAT: Symmetrical radiological inflammation of the sacroiliac joints. WHY: Symmetrical sacroiliitis is a characteristic finding of ankylosing spondylitis, whereas it is less common in Reiter's disease, psoriatic spondylitis, and enteropathic arthritis. In these diseases asymmetric sacroiliitis is not uncommon. HOW: Sacroiliitis is diagnosed by radiological findings of the sacroiliac joints. These findings include loss of the normal subchondral plate, indistinctness of the subchondral plate, narrowing or apparent widening of the sacroiliac joint space, subchondral sclerosis on both the iliac and sacral side of the sacroiliac joint, subchondral erosions of the sacroiliac joint and fusion of the sacroiliac joint.