In a case-controlled study, serum creatinine kinase (CK) activity was significantly lower in 40 patients with RA than in 40 age- and sex-matched patients with non-inflammatory arthropathies [mean 37.6 (S.D. 29.2) vs 77.7 (S.D. 45.3) IU/l respectively P < 0.0001]. In contrast, serum levels of aldolase and myosin were not significantly lower in RA patients. A significant inverse correlation between CK activity and ESR, CRP and platelet count was observed in RA. There was also a positive correlation between haemoglobin levels and CK values. No correlation was found between CK activity and a meager mass index, disease duration and radiological erosion. No inhibitor of CK activity in the sera of RA patients was found. CK serum activity was markedly reduced in RA, and is related to the inflammatory activity of the disease. This finding may stimulate further exploration on the effect of inflammatory response in muscle metabolism.