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Symptoms of ankylosing spondylitis (AS) usually begin between the ages of 15-40 although the disease can also affect children or adolescents as well as adults over 40 years of age. A study of 3000 patients with ankylosing spondylitis from Germany reported that:

• 4% of patients exhibited their first symptoms of AS before the age of 15
• 90% of patients exhibited their first symptoms between the ages of 15 and 40
• 6% of patients exhibited their first symptoms after 40 years of age

Ankylosing spondylitis is three times more common in whites than in African Americans. It has been estimated that approximately 350,000 Americans suffer from ankylosing spondylitis. Internationally, ankylosing spondylitis is thought to occur in up to 1% of the general population with the highest incidence in northern European countries and the lowest occurrence in sub-Saharan Africa. The prevalence of ankylosing spondylitis is about 2 to 3 times higher in males than females with men accounting for about 65% to 80% of cases.

The disease is more severe in men than in women. In men, ankylosing spondylitis primarily affects the spine and the pelvis, whereas, women tend to have more involvement of the knees, hips, wrists, ankles, and pelvis with less severe spinal involvement. Ankylosing spondylitis may appear in children as young as 11 years old - a condition called juvenile ankylosing spondylitis. When AS develops in children or adolescents, it tends to involve the knees, ankles, feet, hips and buttocks. Back pain, however, is very rare.

One of the major problems for individuals suffering from ankylosing spondylitis and for health care providers is the delay between the onset of symptoms and the diagnosis of the condition. It has been estimated that the average time delay between onset of symptoms and definitive diagnosis is about 9 years. The diagnostic delay for women is even longer than for men. Ankylosing spondylitis is commonly underdiagnosed in women since its presentation often differs considerably than that in men. This has led some researchers to believe that ankylosing spondylitis may be more prevalent in women than formerly thought.

Many patients do not seek help early in the disease process because the symptoms may be vague and insidious. Inflammatory back pain may occur 5-10 years before being diagnosed and in that time period, progressive structural damage is taking place. The diagnosis is only established when the damage and structural abnormalities of the sacroiliac joint and/or spine become visible on x-ray images.

It is important not to confuse 'spondylitis' with 'spondylosis'. Whereas spondylosis reflects a condition resulting from 'wear and tear' of the spine more frequently seen in older people, spondylitis is an inflammatory condition that leads to new bone formation and fusion and is seen at a younger age.