OBJECTIVE: There is no "gold standard" to assess disease activity in patients with ankylosing spondylitis (AS). It is known that patients and physicians have different opinions about disease activity. The objective was therefore to investigate on which criteria patients with AS and physicians base their judgement on disease activity. METHODS: A cohort of 203 AS out-patients fulfilling the modified New York criteria included in the ongoing long-term follow-up was analysed. The Assessment in Ankylosing Spondylitis (ASAS) International Working Group has established different domains relevant for outcome in AS. Each domain includes a number of instruments for making assessments, and all these instruments are included in the Outcome in Ankylosing Spondylitis International Study and were made every 6 months for 2 yr. Disease activity from the patient perspective as well as from the physician perspective was analysed using the patient's or the physician's global assessment of disease activity [visual analogue scale (VAS): 0 (best)-10 (worst)] by dichotomizing into "high disease activity" (VAS > or = 6.0) and "low disease activity" (VAS < or = 4.0). Data reduction by principal components analysis (PCA) was performed to distinguish factors capturing correlated instruments. Discriminant analysis with the factor loadings was performed to discriminate between a low and a high disease activity state from both the patient's and the physician's perspective. Multiple regression analysis on the discriminant scores was performed to prioritize the instruments. RESULTS: PCA revealed four factors: spinal mobility, physician assessments, patient assessments and laboratory assessments (Cronbach's alpha 0.52-0.80; explained variance 61%). Discriminant function analysis showed that the factor "patient assessments" was most important (pooled correlation 0.85) in discriminating between a low and a high disease activity state as defined by the patient. The other three factors contributed marginally (pooled correlation <0.30). In contrast, the factors "physician's assessments" (pooled correlation 0.62), "spinal mobility" (pooled correlation 0.52) and "laboratory assessments" (pooled correlation 0.48) contributed most to the physician's perspective. The factor "patient assessments" did not contribute at all (pooled correlation 0.05). Multivariate analysis on the discriminant scores showed that the instruments "pain spine", "BASFI", "pain joints" and "BASDAI fatigue" explained more than 90% of variance in the case of the patient perspective. The instruments "cervical rotation", "swollen joint count", "CRP" and "intermalleolar distance" explained more than 90% of variance in case of physician perspective. CONCLUSION: AS patients rate disease activity on the basis of complaints while physicians rate disease activity on the basis of instruments related to disease severity and inflammation.