Objective. To evaluate in patients with AS (i) the fluctuation in self-reported disease activity (BASDAI), patient global well-being [visual analogue scale (VAS)-global] and spinal pain (VAS-pain) during 2 years of follow-up on a group level and (ii) the clinical relevance of these fluctuations on a patient level. Methods. Dutch patients from the Outcome in AS International Study cohort completed patient-reported outcome measures every 2 months over 2 years. On the group level, mixed linear models were used to analyse whether the outcome measures were constant over time. On a patient level, relevant changes in scores were assessed by the frequency in which changes would exceed predefined cut-off values (>1.0 or >2.0 on a 0-10 scale) during the 2 years of follow-up using 2-, 4-, 6-, 12- or 24-month assessments. Results. Ninety patients [median age 47.3 years (s.d. 11.4), 67.8% male, symptom duration 25.2 years (s.d. 11.3)] were included. On the group level, the outcome measures remained constant over time. However, large fluctuations were found on the patient level. For example, using 2 month intervals, 92% and 69% of the patients had at least one change of >1.0 or >2.0, respectively, in the BASDAI during the 2 year follow-up. When prolonging the interval, the proportion of changes exceeding the cut-off decreased, indicating that information is lost. Similar results were found for the VAS-global and VAS-pain. Conclusion. Substantial fluctuations in BASDAI, VAS-global and VAS-pain were found in individuals over time. With longer intervals, fewer fluctuations were observed, indicating that relevant changes might be missed.