Mobility measurements of spine, chest, hips, and shoulder in 73 adult males with ankylosing spondylitis (AS) were correlated with the duration of disease. Several mobility tests showed a significant correlation with the duration of AS, but after adjustment for age, only rotation of the thoracolumbar spine maintained a highly significant correlation. It is concluded that age has a strong effect on the mobility tests. Rotation of the thoracolumbar spine seems to be a valid measure for AS-specific changes.