The role of the complement system in the pathogenesis of systemic diseases is very ambivalent. In systemic lupus erythematosus (SLE), many abnormalities in the activation of the complement system have been reported. The most important antibodies formed against the complement system in SLE are the ones associated with the C1q component. The aim of this study was to assess separately the anti-C1q antibodies and C1q component in the serum from 65 patients with SLE, then in individuals with (n=33) and without (n=32) lupus nephritis and with active (n=36) and nonactive (n=29) form of the disease (European Consensus Lupus Activity Measurement, ECLAM>3, ECLAM<or=3). This study also aims to look for correlations with other clinical and laboratory parameters. The C1q antibodies were measured by the Enzyme-Linked Immunosorbent Assay (ELISA) test, while radial immunodiffusion according to Mancini was used to measure the C1q complement component. The mean serum levels were 90.89+/-13 IU/ml for anti-C1q antibodies and 145+/-52 mg/l for C1q. The significant difference in C1q antibodies levels was found between individuals with and without lupus nephritis (117.5+/-52 IU/ml vs. 28.2+/-12.2 IU/ml, p=0.0001) and between those with active and nonactive SLE (154.6+/-115 IU/ml vs. 50.6+/-73, p=0.001). C1q complement component was statistically lower in patients with lupus nephritis (144+/-30 mg/l vs. 175+/-50 mg/ml, p=0.002) and in active patients (138+/-40 mg/l vs. 202+/-20 mg/l, p=0.001). If the two parameters are measured together, they seem to have a mirror-like pattern of serum concentration, and they are potential markers of SLE activity and of the presence of lupus nephritis.