The insidious onset of low back pain and/or stiffness is often the initial symptom of ankylosing spondylitis. The hallmark of ankylosing spondylitis is symmetrical sacroiliitis that is often bilateral (Fig. 287-2) . Sacroiliitis develops early but may take 7 to 10 years to become evident by conventional radiography. Pain is anatomically localized over the sacroiliac joints and less commonly radiates down the posterior of the thigh. Patients usually complain of inflammatory back-pain--prolonged spinal stiffness each morning that is relieved only by increased activity or anti-inflammatory