The purpose of this study was the validation of a German translation of the Bath Ankylosing Spondylitis Index (BASDAI).The German translation of the BASDAI was validated using data of 134 patients with ankylosing spondylitis (AS) from a spondyloarthropathy cohort. For validation the BASDAI was compared with parameters assessed by physicians (physicians global, C-reactive protein (CRP), erythrocyte sedimentation rate (ESR), spinal mobility (BASMI), swollen joints, enthesitis (yes or no)), and parameters assessed by patients themselves (pain, function (BASFI) and quality of life (SF-12)). The discriminative validity was analyzed by comparison of BASDAI scores of patients subgroups who have different levels of disease severity. The sensibility to change was analyzed by using follow-up data of a clinical trial on efficacy of the anti-tumor necrosis factor (TNF) alpha agent infliximab in 70 AS-patients with active and severe disease.The German translation of the BASDAI was easy to understand, the translation back to English corresponded well with the original English version. The 6 questions of the BASDAI represent different aspects of disease but correlated also well with each other (r < 0.65). There was a good internal consistency for all 6 questions (Cronbach's Alpha 0.82), strong correlations to the physician's global (r = 0.659), pain (r = 789), function (BASFI; r = 0.752) and to CRP and ESR confirming that the BASDAI assesses a comprehensive picture of patient's disease activity. For subgroups of patients with different levels of disease severity or with different manifestations the BASDAI showed good discriminative properties. The BASDAI was sensitive to change during treatment with infliximab (effect size 1.18).The German translation of the BASDAI is easy to use, reliable, and sensitive to change for the assessment of disease activity in AS.