To evaluate C-reactive protein (CRP) as a potential useful criterion of symptomatic severity of ankylosing spondylitis (AS), we conducted both a cross sectional and a longitudinal (6 week) clinical study in 443 patients with axial involvement in AS. During the 6 weeks of the study, patients received either a placebo or an active nonsteroidal antiinflammatory drug (NSAID). At baseline, CRP was increased in 173 patients (39%). A multivariate analysis in which CRP was the dependent variable and all clinical assessment criteria (pain, range of motion, functional disability, hemoglobin, platelet count) the independent variables showed that range of motion and laboratory signs of inflammation were the most significant variables to explain the CRP values. A similar multivariate analysis conducted on the changes in the variables during the 6 weeks of the study concluded that night pain and laboratory signs of inflammation were the most significant variables explaining the changes in CRP values. The capacity of CRP to discriminate between an active NSAID and a placebo was moderate. This study suggests than an increase in CRP in patients with AS with axial involvement is not a rare phenomenon and might be correlated with the clinical severity of the disease. This outcome measure does not seem to be of great interest in the short term evaluation of fast acting drugs. However, the longterm clinical significance of such an increase in CRP remains to be investigated.