Translation and validation of the Bath Ankylosing Spondylitis Disease Activity Index ( BASDAI) for defining disease status in ankylosing spondylitis (AS) and related diseases.The German version BASDAI-D was translated with only a few modifications. It contains six 10 cm horizontal visual analog scales to measure severity of fatigue, spinal and peripheral joint pain, tenderness and morning stiffness. A total of 318 patients with AS and other spondarthropathies completed the instrument at the beginning and at the end of their medical rehabilitation. Psychometric evaluation was performed separately for patients with AS corresponding to the modified New York Criteria ( SPA-NY, N=211) and for a second group of patients, which fulfilled the early diagnostic criteria for AS or the ESSG criteria of spondylarthropathies (Fd/Spond., N=86).SPA-NY: Factor analysis yielded one factor explaining 54.8% of variance. The sum score with a standard deviation of 1.94 is nearly symmetrical distributed with mean of 4.7. Reliability analysis yielded high internal consistency (Cronbach's alpha=0.83). Corrected item-total correlations are also high and are large ranging from 0.48 to 0.71. Concerning concordant validity the BASDAI-D showed a substantial correlation with the scale bodily pain of the SF 36 (r = -0.72). There are no correlations with objective clinical parameters. Furthermore the assessment of disease activity by the physician correlates significantly, albeit low with the BASDAI-D. Sensitivity to change yields effect sizes up to 0.60. Fd/Spond.: The results for this group are almost identical with SPA-NY (one factor explaining 53.8% variance, mean 4.6+/-1.75; corrected item-total correlations from 0.50 to 0.75; correlation with the scale bodily pain r = -0.62). Analysis for subgroups showed higher scores for women in both groups SPA-NY and Fd/Spond.BASDAI-D ist applicable for patients with AS and likewise for patients fulfilling the early diagnostic criteria for AS or the ESSG criteria of spondylarthropathies. In summary, BASDAID is reliable and valid. Because of its good acceptance and usability BASDAI-D is easy to use in research as well as in clinical practice.