The preliminary core set for endpoints in ankylosing spondylitis (AS) clinical trials includes physical function. The objective of this review was to compare properties and performance of the 2 most widely used measures of functional capacity in AS: the Dougados Functional Index (DFI) and the Bath Ankylosing Spondylitis Functional Index (BASFI). A MEDLINE search was performed covering the years 1988 through April 1998. AS studies were identified and selected if they included the DFI and/or the BASFI. Additional studies were identified by scrutinizing the references cited in the retrieved articles and by correspondence with the authors of the 2 questionnaires. The instruments were examined with respect to development, truth, discrimination, and feasibility. We identified 27 articles dealing with the instruments. Both are valid and reliable measures of functional capacity in AS. While the DFI was sensitive to change in one disease controlling (DC-ART) and in 4 symptom modifying antirheumatic drug (SMARD) clinical trials, the BASFI is not sufficiently tested in these settings. The only direct comparison of both indices in a physical therapy setting confirmed 4 other studies that suggested a better discriminative capacity of the BASFI in physical therapy clinical trials. Two likely reasons for the lack of responsiveness of the DFI in these settings are the distribution of baseline scores showing a tendency towards normal values and the less sensitive scoring system. Discriminative capacity of the BASFI was found to be superior in patients with mild functional disability. It remains to be seen whether the BASFI performs as well as or better than the DFI in SMARD and DC-ART clinical trials.