Both dense iliitis and ankylosing spondylitis can manifest clinically as pain in the lumbosacral region. In the clinic, we find that patients often confuse the two diseases. The following is a brief description of the main characteristics of the two diseases to help identify them. 1. Iliac dense osteitis occurs mostly in young and strong women with bilateral sacroiliac joint lesions, associated with excessive weight bearing, local strain or trauma, after childbirth and genital inflammation. The pain is mild, mostly vague or aching, and occasionally radiates to both buttocks and the posterior or lateral thighs. The pain is mostly intermittent and worsens after standing or walking for a long time. X-rays show a triangular or pear-shaped dense band with clear and uniform margins on the iliac side of the sacroiliac joint, with clear sclerotic margins. Laboratory tests are not specific. HLA-B27 is mostly negative. The disease is associated with genetics, infection and other factors. In addition to lower back pain, there is also stiffness of the vertebral body, limited movement and peripheral joint involvement such as knee and ankle. Imaging shows symmetric joint surface destruction of bilateral sacroiliac joints, sclerosis and blurring of joint edges, narrowing of joint spaces, and even joint fusion or ankylosis. HLA-B27 is mostly positive.