Ankylosing spondylitis (AS) is a type of arthritis characterized by long-term inflammation of the joints of the spine typically where the spine joins the pelvis. Occasionally areas affected may include other joints such as the shoulders or hips. Eye and bowel problems may occur as well as back pain. Joint mobility in the affected areas generally worsens over time.

Although the cause of ankylosing spondylitis is unknown, it is believed to involve a combination of genetic and environmental factors. More than 85% of those affected in the UK have a specific human leukocyte antigen known as the HLA-B27 antigen. The underlying mechanism is believed to be autoimmune or autoinflammatory. Diagnosis is typically based on the symptoms with support from medical imaging and blood tests. AS is a type of seronegative spondyloarthropathy, meaning that tests show no presence of rheumatoid factor (RF) antibodies.

There is no known cure for AS. Treatments may include medication, exercise, physical therapy, and in rare cases surgery. Medications used include NSAIDs, steroids, DMARDs such as sulfasalazine, and biologic agents such as TNF inhibitors.

Approximately 0.1% to 0.8% of all humans are affected with onset typically occurring in young adults. Males and females are equally affected; however, women are more likely than men to experience inflammation rather than fusion.