We investigated the association between serum titers of IgG antibodies against C1q (C1qAb) and clinical and laboratory variables of disease activity in systemic lupus erythematosus (SLE). C1qAb were measured by ELISA in serum samples of 88 patients. Thirty patients (34%) had increased C1qAb titers. No correlation between C1qAb titers and a score for general disease activity was found. However, significant positive correlations were found between C1qAb titers and the presence of several clinical and laboratory variables of disease activity. These included nephritis, dermatitis, hypocomplementemia, antibodies against dsDNA, and circulating immune complexes. A negative correlation was found with neurological disease manifestations. The correlations between C1qAb titers and clinical features indicate that the pathogenetic role for C1qAb in certain disease manifestations of SLE deserves further study.