OBJECTIVE: Adaptation of the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI), the Bath Ankylosing Spondylitis Global Score (BASG), and the Bath Ankylosing Spondylitis Functional Index (BASFI) for defining disease status in ankylosing spondylitis (AS) and related diseases for use in Denmark. METHODS: The BASDAI, BASFI, and BASG were translated into Danish and subsequently field-tested among 17 AS patients for relevance, face, and content validity. Reliability and validity were assessed by administering the new measures and a comparator instrument to 113 AS patients on two occasions. RESULTS: Test-retest reliability was high (>0.90) and the random measurement error was within+/-2.0 for the BASG and within approximately+/-1.5 for BASDAI and BASFI, which is acceptable for most clinical settings. The measures have good internal consistency and are able to discriminate between functional impairment and disease activity according to the Nottingham Health Profile (NHP) and the Stanford Health Assessment Questionnaire (HAQ). CONCLUSION: Danish versions of the BASG, BASDAI, and BASFI are feasible for application in clinical trials and epidemiological studies on AS in Denmark.