Objective. To determine if antibodies to the collagen-like region of C1q (C1q-CLR) are present in the glomerular immune deposits of patients with systemic lupus erythematosus (SLE).

Methods. Kidney tissues were obtained at autopsy, glomeruli were isolated, and glomerular basement membrane fragments were prepared. Antibodies were extracted with low pH or with DNase.

Results. The concentrations of antibodies to C1q-CLR recovered from the glomeruli were ⩾50-fold higher per unit of IgG than that found in the serum or in the serum and interstitial fluid entrained in glomeruli. Antibodies to C1q-CLR were recovered from glomeruli of 4 of 5 patients with proliferative glomerulonephritis at autopsy.

Conclusion. This is the first demonstration that antibodies to C1q-CLR are deposited and concentrated in the renal glomeruli of patients with SLE. These antibodies, thus, have the potential of contributing to the pathogenesis of lupus glomerulonephritis.