Ankylosing spondylitis is a form of joint inflammation (arthritis) that is long-lasting (chronic) and most often affects the spine. Ankylosing spondylitis commonly causes pain and stiffness, with swelling and limited motion in the low back, middle back, neck, hips, chest wall, and heels.

Ankylosing spondylitis is a specific disease within a family of diseases called spondyloarthropathies. The cause of the disease is not known. In severe cases the affected joints in the spine fuse together, causing severe stiffness in the back. Other joints can be stiff and painful, including those in the shoulders, wrists, hands, knees, ankles, and feet. Complications of ankylosing spondylitis may include inflammation of the colored part of the eye (iris), called iritis, or difficulty breathing due to curving of the upper body and stiffening of the chest wall. Inflammation from the condition may also affect the heart valves. In rare cases of severe disease, the artery called the aorta, the lungs, the kidneys, and the digestive tract can also be affected.

Ankylosing spondylitis usually affects people younger than about 35 and can run in families. It is more common in men than in women.

Although there is no cure, treatment can usually control symptoms and prevent the condition from getting worse. Most people are able to continue to work and do normal daily activities.