The aim of the study was to investigate the reliability and the validity of the Swedish version of the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI). A total of 113 patients with ankylosing spondylitis were assessed with the BASDAI, the Swedish 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 a questionnaire on their opinions of the relevance of the BASDAI. The test-retest stability investigation of the BASDAI over 24 hours did not show any difference between the two occasions (md 4.4, range 0.80-8.43 vs md 4.0, range 0-7.80, p > 0.05). The correlation coefficient between the BASDAI and the BASMI was r(s) = 0.07, between the BASDAI and the BASFI r(s) = 0.64, and between the BASDAI and the BAS-G r(s) = 0.80. Eighty percent of the patients considered the contents of the BASDAI to be relevant. The BASDAI, the BAS-G and the BASMI showed significant improvements after an intensive rehabilitation period. In conclusion the results of the present study indicate that the Swedish BASDAI is reliable and valid.