Serum C-reactive protein (CRP) levels were measured in 99 patients with definite or classical rheumatoid arthritis (RA). At the same time as the serum was obtained, the activity of the RA was assessed by assigning scores to a comprehensive set of subjective, semi-objective and objective clinical features. The results confirmed that the serum CRP concentration closely reflects activity of RA and is of value in its objective assessment. Erythrocyte sedimentation rate (ESR) followed the same pattern as CRP levels but all of the subjective and semi-objective criteria correlated more significantly with CRP levels than with ESR.