Experts don't know exactly how common ankylosing spondylitis is, because the condition is often not diagnosed. The prevalence (how common it is) differs among groups of people.

Ankylosing spondylitis is more common in men than in women. It is often found in certain ethnic groups, such as Inuits of Alaska and Siberia. It is much less common in people whose family is Japanese or African.¹

The only clear risk factor for ankylosing spondylitis is having a close family member who has the condition. About 15 to 20 out of 100 people with ankylosing spondylitis have at least one family member who has it.¹ Having a gene known as HLA-B27 and having frequent infections of the gastrointestinal system (stomach and other organs of digestion) may also increase your risk.