In the absence of an ideal objective measure for assessing ankylosing spondylitis (AS), self-administered measures of disease activity (the Bath Ankylosing Spondylitis Disease Activity Index, BASDAI) and function (the Bath Ankylosing Spondylitis Functional Index, BASFI) have been developed, in addition to an objective measure of spinal mobility (the Bath Ankylosing Spondylitis Metrology Index, BASMI). However, a more global assessment is also desirable. We report on the design and validation of a global measure (the Bath Ankylosing Spondylitis Patient Global Score, BAS-G) which reflects the effect of AS on the patient's well-being. A pilot study was performed to select the most appropriate wording for BAS-G. Using 392 patients with AS, BAS-G's construct and predictive validity and test-retest reliability were assessed. Correlations between BAS-G and BASDAI/BASFI were calculated, and multiple regression was used to examine the significant correlates. The distribution of the responses covered the whole scale. As predicted, BAS-G correlated best with BASDAI (r=0.73), followed by BASFI (r=0.54). The best fitting regression equation included these scales as well as patients' gender and current age. One week and 6 month scores were significantly different (P<0.001). Construct validity was good: BAS-G correlated more strongly with each component of BASDAI and BASFI than with BASMI or with gender. Predictive validity was satisfactory: there was an improvement (mean=29%) in in-patient BAS-G scores over a 2 week treatment period (P<0.001). Test-retest reliability was excellent (1 week r=0.84, 6 months r=0.93). BAS-G correlates well with both BASDAI and BASFI, suggesting that disease activity and functional ability play a major role in patients' well-being, whereas metrology does not. The score is sensitive to change, reliable, and meets face, predictive and construct validity criteria.