The aim of this study was to develop a Turkish version of the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) and assess its reliability, validity, and sensitivity to change. The Turkish version was obtained after a translation and back-translation process. Seventy-one patients with ankylosing spondylitis were assessed with it. To assess its validity, patients were also evaluated with the Turkish version of the Bath Ankylosing Spondylitis Functional Index (BASFI), the Bath Ankylosing Spondylitis Metrology Index (BASMI), the Bath Ankylosing Spondylitis Patient Global Score (BAS-G), and physicians' assessments of disease activity. Over 24 h, the Turkish BASDAI did not show significant differences between the two assessments (P>0.05). Correlations were found between BASDAI and all of the evaluation parameters except BASMI (P<0.01). Both BASDAI and BASMI showed significant improvements after 8-week home exercise programmes. The results of this study show that the Turkish version of the BASDAI is reliable, valid, and sensitive to change.