OBJECTIVE Antibodies to C1q (aC1q) have been found in up to 50% of adult patients with systemic lupus erythematosus (SLE) and have been associated with proliferative glomerulonephritis. We investigated the prevalence and clinical significance of aC1q in pediatric SLE. METHODS Antibodies to C1q, measured by an ELISA method, were evaluated in 29 patients with childhood-onset SLE, 26 females and 3 males, aged 7.5 to 19.6 years. RESULTS Seventeen (59%) of the 29 patients were initially positive for aC1q. No correlation was found between either the presence or titer of aC1q and any of the clinical manifestations, including nephritis. However, a significant correlation was observed between aC1q levels and anti-double-stranded DNA antibodies and C3 values. Serial determinations of aC1q in 23 patients showed a progressive decline in the titers with few significant fluctuations. Antibodies to C1q were not a reliable predictor of increased SLE activity, since they increased or became detectable in only 50% of the pre-flare sera. CONCLUSIONS Antibodies to C1q were frequently positive in our patients with pediatric SLE and were correlated with laboratory variables of disease activity. However, unlike in adults with SLE, a high correlation between aC1q and glomerulonephritis was not found in these pediatric patients.