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Disease characteristics of Ankylosing Spondylitis Definition of the disease Ankylosing spondylitis (AS) is a relatively common chronic inflammatory disorder that more often manifests in young males than in females. The disease presents with low back pain and morning stiffness due to a chronic inflammation of the sacroiliac (SI) joints and vertebral column. This inflammatory process can result in destruction of the vertebral column leading to postural deformities, like ankylosis of the cervical spine and kyphosis of the thoracic spine. Extraspinal manifestations of the disease consist of arthritis of the peripheral joints (especially knees, shoulders and hips), resulting in joint destruction that sometimes necessitates joint replacement, anterior uveitis, enthesitis, cardiac and pulmonary complications 1,2. The diagnosis of definite AS requires fulfilment of the modified New York criteria3: obligatory are signs of a bilateral sacroiliitis grade 2-4 or unilateral sacroiliitis grade 3 or 4 plus at least one criterion out of 3 (inflammatory back pain, limited lumbar spinal motion in sagittal and frontal planes and decreased chest expansion relative to normal) The onset of complaints is often gradual and the mean delay is 8 years to the time o