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Supplemental Digital Content is available in the text Abstract The aim of this study was to investigate the relationship between long-term spinal mobility improvements and early disease activity changes or achievement of clinical response criteria in patients with ankylosing spondylitis (AS) treated with tumor necrosis factor (TNF-α) blockers. This retrospective study included 112 patients with AS treated with TNF-α blockers for up to 33 months. The paired t-test was used to compare outcome measures between visits. The correlation between disease activity changes and metrological improvements was analyzed using cumulative probability plots, Spearman correlation coefficient, and canonical correlation. The difference in metrological outcomes between responders and non-responders to clinical response criteria was also examined. Metrological and disease activity outcomes improved most markedly in month 3. All disease activity outcomes and ESR from baseline to month 3 (3-month) were significantly correlated with the Bath Ankylosing Spondylitis Metrology Index (BASMI10) improvements from baseline to month 33 (33-month). The 3-month changes in ankylosing spondylitis disease activity score (ASDAS-CRP) and patient's global assessment showed a significant correlation with the 33-month changes in chest expansion. Only responders according to ASDAS major improvement at month 3 demonstrated significant 33-month improvements in both BASMI10 and chest expansion, compared to non-responders. Responders according to Assessment of Spondylo Arthritis international Society 40 at month 3 showed significant 33-month improvements in BASMI10, but not chest expansion, compared to non-responders. The degree of early changes in disease activity outcomes influenced the extent of long-term metrological improvements in AS treated with TNF-α blockers. Additionally, the achievement of ASDAS- major improvement at month 3 predicted significant metrological improvements throughout long-term TNF-α-blocker therapy.