The aim of this study was to adapt the Bath Ankylosing Spondylitis Functional Index (BASFI) to the Turkish population and investigate the reliability and the validity of the Turkish version. Seventy-six patients with ankylosing spondylitis (AS) were included in the study. The functional status of the patients was assessed by using the adapted Turkish version of the BASFI twice, at recruitment and 24 h later. For validity analysis, patients were also assessed by the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) evaluating disease activity, the Bath Ankylosing Spondylitis Global Score (BAS-G) indicating effect of the disease on patient's well-being, physician's assessment of the disease activity and pain intensity. Spinal mobility was assessed by the Bath Ankylosing Spondylitis Metrology Index (BASMI). Erythrocyte sedimentation rate (ESR) and serum C-reactive protein (CRP) levels of the patients were also recorded. The lumbar region and the sacroiliac joints were assessed by Stoke Ankylosing Spondylitis Spine Score (SASSS) and the hip joints were assessed by Bath Ankylosing Spondylitis Radiology Index hip (BASRI-h). The internal consistency was 0.89 (Cronbach's alpha), which showed a high reliability for the Turkish version of the BASFI. Test-retest reliability was good, with a high intraclass correlation coefficient between the two time points (ICC=0.93). Significant correlations were detected between the BASFI and the BASDAI, BAS-G, doctor's global assessment, and general pain intensity (r=0.62, p<0.001; r=0.47, p<0.001; r=0.55, p<0.001; r=0.47, p<0.001, respectively). The adaptation of the BASFI to the Turkish population was successful and it was found to be reliable and valid among Turkish patients. Thus, studies using the Turkish BASFI can be compared with international studies.