Previous studies have suggested that serum Clq levels measured during the first 5 years of rheumatoid arthritis (RA) may be predictive of the extent of subsequent joint damage. To further evaluate the clinical significance of this marker in RA, levels of Clq were measured by radial immunodiffusion in serum samples from 107 well-characterized patients with RA. Mean levels of Clq were higher in patients with a disease duration 5 5 years (173 pg/ml) than in patients with a disease duration >5 years (148 pglml) (P = 0.032).Serum Clq levels were correlated with total joint counts and activities of daily living scores, but no correlation was observed with erythrocyte sedimentation rates or with radiographic scores. The results suggest that Clq may be a useful early marker of disease activity in patients with RA.