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Assessment of spinal and hip mobility has been recommended by the Assessments in Ankylosing Spondylitis (AS) Working Group for clinical trials and record keeping, although suggested measures primarily reflect structural damage. Our objective was to validate a simple, 4-item composite measure of spinal and hip mobility, the Edmonton AS Metrology Index (EDASMI).We assessed the EDASMI and the Bath AS Metrology Index (BASMI) using a total of 263 patients from 3 countries: Canada (n = 205), Australia (n = 29), and Colombia (n = 29). Intra- and interobserver reliability were assessed in a subset of 44 patients. Construct validity with respect to disease activity (Bath AS Disease Activity Index [BASDAI]), function (Bath AS Functional Index [BASFI]), and structural damage (modified Stoke AS Spinal Score [mSASSS]) was analyzed using correlation and hierarchical regression. Responsiveness was assessed in a subset of 33 patients who received either anti–tumor necrosis factor α therapy (n = 26) or pamidronate (n = 7) over 24 weeks.In contrast to the EDASMI, BASMI scores covered a limited range, with 70% of patients demonstrating a score ≤3 (range 0–10) and 4 of 5 individual measures demonstrating substantial floor effects. Both measures were highly reliable (intraclass correlation coefficient >0.90) and demonstrated similar construct validity (EDASMI correlated with disease duration [0.52], BASDAI [0.24], BASFI [0.61], Bath Ankylosing Spondylitis Radiology Index [0.79], mSASSS [0.75]; P < 0.001 for all). The change in EDASMI score was significant after 24 weeks of therapy (standardized response mean 0.40; P = 0.03), but change in the BASMI was not significant.The EDASMI is a simple, rapid, and reliable tool for the assessment of spinal mobility in AS that is responsive to therapeutic intervention.