Although ankylosing spondylitis does not necessarily occur in those who are HLA-B27 positive and does not necessarily not occur in those who are HLA-B27 negative, HLA-B27 positivity is an important reference for the diagnosis and prognosis of ankylosing spondylitis. (1) HLA-B27 positivity significantly increases the chance of a correct diagnosis if symptoms and signs suggest spondyloarthropathy. (2) In children with inflammatory arthropathy, a positive HLA-B27 may suggest the possibility of ankylosing spondylitis. (3) If the children of patients with ankylosing spondylitis are HLA-B27 positive, especially in males, it suggests a higher likelihood of developing ankylosing spondylitis. In addition, a study by Liu Xiangyuan et al. found that compared with HLA-B27-negative ankylosing spondylitis, HLA-B27-positive patients had relatively early onset, more severe clinical symptoms, higher incidence of systemic symptoms and peripheral arthritis (e.g., hip), more altered immunological indicators (increased ESR, CRP, and γ-globulin), and a higher incidence of familial aggregation, bamboo-like spine, and ophthalmoplegia In addition, HLA-B27-positive patients have a more severe disease and a poor prognosis. In conclusion, HLA-B27 is a reference indicator for the diagnosis of ankylosing spondylitis, but not a basis for diagnosis. If the diagnosis of ankylosing spondylitis has been confirmed by x-ray, HLA-B27 may not be performed.