Human Leukocyte Antigen B27 (HLA B27) is a surface antigen strongly associated with spondyloarthropathy (joint disease in the spine). HLA B27 is most often associated with ankylosing spondylitis (AS), but is also associated with several other problems.

Up to 90% of patients with clinical findings of AS have a positive HLA B27, however less than 20% of HLA B27 positive patients develop AS. This reinforces the importance of clinical diagnosis rather than dependence on the test as diagnostic criteria.