The cause of this disease is still unknown, but it is generally believed that genetic, environmental, infectious, and immune factors interact to cause it. (A) Genetic factors Ankylosing spondylitis often occurs in more than two members of a family. The risk of ankylosing spondylitis in the first generation of relatives of patients with ankylosing spondylitis is 20 to 40 times higher than in the general population. A survey of twin children found that in monozygotic twins, the likelihood of the other one developing the disease was more than 50%. (B) Infectious factors Recent studies have suggested that the occurrence of the disease may have days with infection; patients with ankylosing spondylitis have a 79% positive stool culture for Klebsiella pneumoniae, compared with 30% in the normal population, indicating that the frequency of Klebsiella pneumoniae infection is significantly higher in patients than in the normal population. Serum anti-Klebsiella pneumoniae antibody levels were also found to be significantly higher in patients with ankylosing spondylitis, with a positivity rate of 43.3% compared to 4.4% in normal subjects. The better efficacy of the treatment of ankylosing spondylitis with sulfasalazine further supports the relationship between intestinal infection and ankylosing spondylitis. （The HLA-B27 gene belongs to MHC class I. According to the DNA typing method, 15 subtypes have been identified so far. Epidemiological data suggest that ankylosing spondylitis is positively associated with B2704, B2705 and B2702, and negatively associated with B2709 and B2706, which may be due to differences in the amino acid sequence of certain parts of the B27 molecule. (iv) Other factors Age, body mass, malnutrition, climate, soil, moisture and cold. Others include trauma, parathyroid disease, upper respiratory tract infection, and local septic infection, which may have some relationship with the disease, but the evidence is insufficient.