After pain and stiffness, one of the most important complaints of patients with ankylosing spondylitis (AS) is disability. The main aims of treatment are to control pain but also to improve function. Various methods of assessing function exist but are either not specific for the disease or have not been adequately validated. As a result of this deficiency we developed the Bath Ankylosing Spondylitis Functional Index (BASFI) as a new approach to defining and monitoring functional ability in patients with AS.