The aim of the study was to investigate the reliability and the validity of the Swedish version of the Bath Ankylosing Spondylitis Functional Index (BASFI). A total of 113 patients were assessed with the BASFI, the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI), the Bath Ankylosing Spondylitis Metrology Index (BASMI), and the Bath Ankylosing Spondylitis Patient Global Score (BAS-G). The median BASFI scores on two occasions within 24 hours were 3.6 versus 3.7 (p>0.05). The patients found the BASFI items relevant. The median self-reported and physiotherapist-observed BASFI scores were 3.4 and 2.8 respectively (p>0.05). The correlation coefficient between the BASFI and the BASMI was r(s)=0.55, between the BASFI and the BASDAI r(s)=0.68, and between the BASFI and the BAS-G r(s)=0.67. Significant improvements between the pre- and post-training results for both the BASFI (3.1 vs 2.0, p<0.001) and the BASMI (3.0 vs 1.0, p<0.001) were found after three weeks' inpatient rehabilitation. The results indicated that the Swedish BASFI is reliable and valid.