Five out of 6 patients with systemic lupus erythematosus (SLE) and chronic polyarthritis were found to have significantly raised (greater than 20 mg/l.) C-reactive protein (CRP) levels. In contrast, slight elevations in CRP levels were present in only 5 of 16 SLE patients without chronic polyarthritis. Thus, an elevated CRP level might be a distinctive feature of SLE patients with chronic polyarthritis.