['Serum concentration of antibodies to C1q (C1qAb) has been reported to be elevated in a high percentage of patients with systemic lupus erythematosus (SLE). The associations of high C1qAb levels with different clinical manifestations and the activity of the disease, however, are not definitely understood.', 'We measured the levels of IgG type C1qAb in the sera of 137 patients with SLE using an ELISA method.', 'Serum concentrations of C1qAb were found to be higher (p < 0.0001) in SLE patients than in healthy controls. High titer (> 66 AU/ml) C1qAb was found in 40/137 (29.2%) SLE patients, and 4/192 (2.1%) healthy controls (p < 0.0001). A strong negative correlation (R = -0.4, p < 0.0001) between the age of the patients and the C1qAb titers could be detected. C1qAb levels in clinically active SLE patients significantly (p < 0.0001) exceeded those measured in the sera of patients in the inactive stage of the disease. A significant positive correlation was detected between C1qAb levels and the laboratory activity markers (anti-DNA, low C3 level) of the disease. We found a significant negative correlation between levels of C1qAb and a negative acute phase protein, alpha2-HS-glycoprotein. Renal involvement was present in 11/40 (27.5%) and 11/97 (11%) of the patients with high and low titers of C1qAb, respectively (p = 0.038). The prevalence of other organ manifestations was, however, the same in the patients with or without high titer C1qAb.'] Are high levels of antibodies against Clq associated with disease activity and nephritis but not with other organ manifestations in SLE patients?