The study objective was to examine natural variation of the patient-reported outcome measures fatigue, pain, patient global assessment (PaGl) and the Bath Ankylosing Spondylitis Functional Index (BASFI) in patients with stable axial spondyloarthropathy (ax-SpA) defined on the basis of the Bath Spondylitis Ankylosing Disease Activity Index (BASDAI). 107 TNF-inhibitor treated stable ax-SpA patients were identified in the Danish rheumatology registry (DANBIO). According to the Assessment of SpondyloArthritis international Society (ASAS) response criteria, stable disease was defined as a change in BASDAI < 20 between two consecutive visits. Data on BASDAI, fatigue, pain, PaGl and BASFI (0-100) from such two visits were extracted for each patient. Lower and upper 95% limits of agreement (LLoA;ULoA) and the mean of intra-individual differences (the bias) were computed for each measure. Associations were described by linear correlations and standard errors of estimation. Mean BASDAI was 35.6 ± 23.8, mean BASDAI change 0.0 ± 9.7 (range - 19 to 19) and mean inter-visit time duration 16 ± 13 weeks. LLoA;ULoA [bias] for fatigue was - 37.4;36.2 [- 0.6], for pain - 34.1;32.5 [- 0.8], for PaGl - 35.7;32.9 [- 1.4] and for BASFI - 23.2;22.6 [- 0.3]. Intra-individual differences in fatigue, pain, BASFI and PaGl were not correlated with the inter-visit time duration, were poorly inter-correlated and were poorly correlated with baseline values and with changes in BASDAI. In conclusion, natural variation of patient-reported outcome measures was substantial and unpredictable in individual ax-SpA patients in steady state defined on the basis of BASDAI. Consequently, observed changes in the daily clinic should be interpreted with caution.