BACKGROUND The results of recent studies suggest that there is a link between the presence of antibodies against C1q (anti-C1q Abs) and kidney involvement in systemic lupus erythematosus (SLE). However, it remains unclear whether the clinical symptoms of lupus nephritis (LN) may be associated with the presence of anti-C1q Abs in serum. OBJECTIVE The aim of the study was to compare the prevalence and levels of anti-C1q Abs and antibodies against double-stranded DNA (anti-dsDNA Abs), circulating immune complexes binding C1q (CIC-C1q), as well as complement components C3 and C4 in the sera of patients with LN in relation to the clinical activity of SLE and symptoms of LN. METHODS The study involved 48 patients with LN and 66 healthy controls. Anti-dsDNA Abs, anti-C1q Abs, and CIC-C1q levels were determined by immunoenzymatic methods, while C3 and C4 by immunoturbidimetry. SLE activity was assessed using the Systemic Lupus Erythematosus Disease Activity Index (SLEDAI). RESULTS Anti-dsDNA Abs, anti-C1q Abs, and CIC-C1q were detected in 77%, 60%, and 43.7% of the patients with LN, respectively. The prevalence and mean levels of anti-dsDNA and anti-C1q Abs were significantly higher in patients with active LN than in those with inactive LN or controls. The levels of C3 and C4 were significantly lower in active LN than in inactive LN or controls. In active LN, a positive correlation between anti-C1q and anti-dsDNA Abs was observed. In patients with detected anti-C1q Abs, microhematuria (59% vs. 16%, P = 0.003), urinary casts (28% vs. 8%, P = 0.02), and low levels of serum C3 (P = 0.03) and C4 (P = 0.01) were observed statistically significantly more often. CONCLUSIONS Simultaneous presence of hematuria and anti-C1q Abs may indicate an ongoing inflammatory process in the glomeruli in patients with SLE.