Our aim was to determine whether C-reactive protein (CRP) or erythrocyte sedimentation rate (ESR) is more appropriate in measuring disease activity in ankylosing spondylitis (AS). We studied 191 consecutive outpatients with AS in The Netherlands, France, and Belgium. Patients were attending secondary and tertiary referral centers. The external criterion for disease activity was: physician and patient assessment of disease activity on a visual analog scale (VAS) and the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI). In each measure we defined 3 levels of disease activity: no activity, ambiguous activity, and definite disease activity. The patients with AS (modified New York criteria) were divided into 2 groups: those with spinal involvement only (n=149) and those who also had peripheral arthritis and/or inflammatory bowel disease (IBD) (n=42). For each criterion of disease activity, the patients with no activity and with definite activity were included in receiver operator curves and used to determine cutoff values with the highest sensitivity and specificity. We also calculated Spearman correlations. The median CRP and ESR were 16 mg/l and 13 mm/h, respectively, in the spinal group and 25 mg/l and 21 mm/h, respectively, in the peripheral/IBD group. In both groups the Spearman correlation coefficients between CRP and ESR were around 0.50. There was moderate to poor correlation between CRP, ESR, and the 3 disease activity variables (0.06-0.48). Sensitivity for both ESR and CRP was 100% for physician assessment and between 44 and 78% for patient assessment of disease activity and the BASDAI, while specificity was between 44 and 84% for all disease activity measures. The positive predictive values of CRP and ESR in our setting were low (0.15-0.69). We conclude that neither CRP nor ESR is superior to assess disease activity.