Ankylosing spondylitis is a chronic inflammatory disease that typically affects the spine. The disease is a form of arthritis that affects an estimated 350,000 people in the United States. It often causes back pain and stiffness. Ankylosing spondylitis may also affect other parts of the body such as the hips, shoulders, and knees. The disease is more likely to affect men than women. The cause or causes of ankylosing spondylitis are not yet known, but scientists have identified certain factors that may increase a person's risk for the condition. Although ankylosing spondylitis can have serious effects on a person's life, current treatment strategies for the condition allow most people with the disease to lead active and productive lives.