This study will evaluate: 1) whether computed tomography (CT) scanning is better than regular x-rays for measuring changes in the stiffness, or fusion, of the spine in patients with ankylosing spondylitis; and 2) if CT can be used to determine how fast extra bone forms in the spine of these patients. Better ways to measure spinal fusion are needed to be able to evaluate the effectiveness of medicines in slowing or stopping its progression. CT uses x-rays to provide detailed pictures of the inside of the body and are valuable for detecting spinal abnormalities because of the precision with which it can show these structures. For the procedure, the patient lies on a table that moves into a large, donut-shaped scanner that can move around the body to take pictures at different angles, which are viewed on a computer monitor.||Patients 18 years of age and older with ankylosing spondylitis who are not currently taking or planning to receive treatment with anti-TNF alpha medications (etanercept, infliximab, adalimumab) for 1 year may be eligible for this study. Participants have eight clinic visits, scheduled at study entry and at 4, 8, 12, 16, 20, 24, and 48 months, at the NIH Clinical Center for the following procedures:||Clinical assessment (all visits) - includes medical history and physical examination, measurement of spine flexibility with a tape measure and protractor, symptoms questionnaire|Blood tests for measures of inflammation, including red blood cell sedimentation rate and C-reactive protein level (all visits)|Urine pregnancy test in women of child-bearing age (visits 1, 4, 7, 8)|X-rays of the pelvis, low back, and neck (visits 1, 4, 7)|X-ray of the low back (visit 8)|CT scan of the low back (visits 1, 4, 7)|Magnetic resonance imaging (MRI) of the low back (visits 1, 4) - MRI combines a powerful magnet with an advanced computer system and radio waves to produce accurate, detailed pictures of organs and tissues. The patient lies on a table in a narrow cylinder containing a magnetic field, wearing earplugs to muffle loud noises that occur with electrical switching of the magnetic fields. He or she can speak with a staff member via an intercom system at all times during the procedure. During the scan, a contrast dye (gadolinium) is injected into the bloodstream through a catheter (plastic tube inserted in a vein) to brighten the images.||In addition, participants will complete a symptoms questionnaire by mail every 4 months for 2 years between visits 7 and 8.