What are the causes of ankylosing spondylitis? There is no clear answer as to how ankylosing spondylitis occurs, although numerous studies have found that its development is mainly related to genetic, infectious, immune, environmental and traumatic, endocrine, and metabolic disorders. The first is that ankylosing spondylitis often occurs in more than two members of a family. First-degree relatives of patients with ankylosing spondylitis have a 20- to 40-fold higher risk of developing the disease than the general population. If one of the monozygotic twin brothers and sisters has the disease, there is a 50% or greater chance that the other will develop the disease. In the past 20 years, a large number of studies have proven that the onset of ankylosing spondylitis and human leukocyte antigen HLA-B27 are closely related, and the HLA-B27 positivity rate in our normal population is 2% to 7%, while the HLA-B27 positivity rate in patients with ankylosing spondylitis is 91%. Other data show that the prevalence of ankylosing spondylitis is about 0.1% in the general population, up to 4% in the family line of ankylosing spondylitis patients, and up to 11%-25% in the first-degree relatives of HLA-B27-positive ankylosing spondylitis patients. These data suggest that HLA-B27-positive individuals, or those with a family history of ankylosing spondylitis, have a significantly increased likelihood and risk of developing ankylosing spondylitis. Second, recent studies suggest that the development of the disease may be related to infection. Patients with ankylosing spondylitis have a significantly higher frequency of K. pneumoniae infection in stool cultures than normal subjects (79% in the former and 30% in the latter). Serum levels of anti-Klebsiella pneumoniae antibodies were significantly higher in patients with ankylosing spondylitis, with a positivity rate of 43.3% compared with 4.4% in normal subjects. Using colonoscopy, inactive enterocolitis was found in 29-49% of patients with ankylosing spondylitis, and the incidence of abnormal lesions was even higher when observed with histology. Salazosulfapyridine, which has anti-infective activity, has shown good efficacy in the treatment of ankylosing spondylitis. All of these suggest a relationship between intestinal infections and ankylosing spondylitis.