During clinic hours, we often encounter many patients who are referred from primary care hospitals with a note that says “HLA-B27 test requested”. After talking to the patients, I realized that the primary care physicians told the patients that if the HLA-B27 is positive, it must be mandatory (AS)! Is that really true? Let’s see what is HLA-B27 and is it this amazing? H stands for Human (Human), L stands for Leucocyte (Leucocyte), and A stands for Antigen (Antigen), which is Human Leukocyte Antigen (HLA). The 8th International Conference on Histocompatibility identified 92 HLAs belonging to 5 loci, A, B, C, D and DR, called HLA-A, HLA-B, HLA-C, HLA-D and HLA-DR, respectively. 42 HLA-B loci are identified, of which B27 is one. HLA-B27 antigen positivity alone cannot be used as a means of diagnosing or confirming AS. HLA-B27 is inherited from the parents, is carried for life, and does not turn negative with treatment. Those who are HLA-B27 positive, or have a family history of ankylosing spondylitis, have a significantly increased likelihood and risk of developing ankylosing spondylitis. The development of ankylosing spondylitis and HLA-B27 have been shown to be closely related, and there is a clear family tendency to develop the disease. If the children of a patient with ankylosing spondylitis are HLA-B27 positive, especially if they are male, it suggests that they are more likely to develop ankylosing spondylitis. However, a positive HLA-B27 does not necessarily mean ankylosing spondylitis, nor is HLA-B27 necessarily positive in patients with ankylosing spondylitis. The vast majority of HLA-B27-positive individuals do not develop ankylosing spondylitis. HLA-B27 positivity is only one factor that predisposes to the development of ankylosing spondylitis. In conclusion, HLA-B27 is a reference indicator for the diagnosis of ankylosing spondylitis and is not a basis for diagnosis. If the signs and symptoms suggest that the patient has ankylosing spondylitis, a positive HLA-B27 significantly increases the chances of a correct diagnosis.