OBJECTIVE The Bath Ankylosing Spondylitis Metrology Index (BASMI) is an index comprising 5 measures of spinal and hip mobility in AS that has been primarily validated in the setting of a physiotherapeutic intervention and has not been validated in relation to functional outcomes. Our aim was to validate the BASMI and its individual components in relation to a validated functional index, the Bath AS Functional Index (BASFI), and to assess its responsiveness in patients with AS receiving 60 or 10 mg pamidronate monthly for 6 months in a double blind, randomized, controlled trial. METHODS AS patients were assessed with the BASMI and BASFI at baseline and 6 months. Two versions of the BASMI were evaluated: the original scoring was based on a 0-2 score for each measure, while a newer version scores each measure on a 0-10 scale. Paired t tests, effect sizes (ES; mean difference divided by baseline standard deviation), and standardized response means (SRM; mean difference divided by standard deviation of the difference) were used to assess responsiveness. A value > 0.5 was considered to reflect good responsiveness. Responsiveness was also examined by linear regression analysis adjusting for age, sex, disease duration, and baseline Bath AS Disease Activity Index (BASDAI) and BASFI. The contribution of the BASMI to the variance in the BASFI was assessed by hierarchical linear regression analysis, independent variables being entered in the following order: (1) age, sex, disease duration; (2) drug dose group; (3) baseline or change in the BASDAI; and (4) change in the BASMI. Pearson correlation coefficient analysis was also performed to examine the contribution of individual measures of the BASMI. A value > 0.6 was defined as indicative of a good correlation. RESULTS Seventy AS patients completed 6 months of therapy [81% male, mean age 40.3 yrs (SD 9.7), disease duration 15 yrs (SD 9.5)]. The responsiveness of the BASMI was poor regardless of which version of the BASMI was used (ES = 0.26, SRM = 0.47 for the 60 mg dose group and using a 0-10 scoring system). Examination of the responsiveness of the individual components of the BASMI revealed significance for lumbar side flexion in the 60 mg dosing group (ES = 0.4, SRM = 0.43; p = 0.01) using the newer version of the BASMI (0-10 scoring) that was of similar responsiveness to the entire BASMI. Linear regression analysis showed no significant effects of age, sex, disease duration, baseline BASFI and BASDAI, or drug dose group on BASMI change scored using a 0-2 grading, but drug dose group had a significant effect on BASMI change scored using a 0-10 grading (p = 0.04). The correlation between changes in the BASMI and the BASFI was low, although significant when either version of the BASMI was examined (0.44 and 0.46 for the 0-2 and 0-10 scoring systems, respectively; p < 0.001). Of the individual components of the BASMI, a significant association was observed between changes in either cervical rotation or lumbar side flexion and changes in the BASFI (0.44 and 0.37, respectively; p < 0.01). After adjusting for age, sex, disease duration, the baseline BASDAI, and drug dose group, the BASMI added significantly to the variance in the BASFI (p < 0.001), but this was no longer significant once change in the BASDAI was added to the regression model. CONCLUSIONS Responsiveness of the BASMI was poor with either scoring system. Lumbar side flexion was the most responsive of the BASMI components. Changes in the BASMI and its individual components did not correlate well with changes in functional outcomes.