Ankylosing spondylitis is a type of arthritis that affects the spine. This is a chronic inflammatory arthritis that commonly affects the sacroiliac joints in young men. The onset is usually rapid with recurring episodes of low back pain and stiffness, which radiates sometimes to buttocks or thighs. Characteristically, the symptoms are worse in the early morning and following inactivity. The vertebrae may grow or fuse together resulting in a rigid spine. These changes may be mild or severe and may lead to a stooped posture. Early diagnosis and treatment helps in controlling pain and stiffness and may prevent significant deformity. Although it can occur at any age, ankylosing spondylitis most often strikes men in their teens and twenties.