Serum creatine kinase (CK) activity is reduced in some conditions, including rheumatic diseases, but the aetiology and significance remain to be clarified. The aim of this study was to investigate relationships between serum CK activity and other muscle enzymes, muscle mass, renal function, steroid use and disease activity in patients with rheumatic diseases. Serum CK activity was measured in sera from 498 patients with rheumatic diseases: rheumatoid arthritis (RA, n = 145), systemic lupus erythematosus (SLE, n = 31), spondyloarthropathies (SpA, n = 35), polyarthralgia/arthritis (Poly, n = 74), miscellaneous group (MI, n = 46), and in non-inflammatory arthropathies (NIA, n = 167) as controls. Serum CK level was significantly reduced in RA (45.4 +/- 1.9 IU/l), SLE (46.4 +/- 4.2 IU/l), SpA (64.7 +/- 5.6 IU/l) and MI (63.4 +/- 4.8 IU/l), but not in poly (70.2 +/- 3.1 IU/l), compared to controls (78.9 +/- 2.4 IU/l) (P < 0.05). CK values correlated with aspartate aminotransferase (AST), erythrocyte sedimentation rate (ESR), body mass index (BMI) and platelets (Plat) in RA; ESR and haemoglobin (Hb) in SLE, AST, ESR and Hb in SpA; lactate dehydrogenase (LDH), AST, ESR and Hb in Poly; LDH, AST, ESR, Hb and Ccr in MI; and LDH, AST and ESR in controls. In all patients with rheumatic diseases CK level was significantly correlated with LDH, AST, alanine aminotransferase (ALT), ESR, C-reactive protein (CRP) and BMI and prednisolone dose, but not with Ccr, age and disease duration. In conclusion, our data support the possibility that reduced CK activity is inversely correlated with inflammatory activity and correlated with other muscle enzymes, muscle mass and steroid use, but not with renal function, age and disease duration in rheumatic diseases.