Autoantibodies to complement components are associated with various diseases. Anti-C1q antibodies are present in all patients with hypocomplementemic urticarial vasculitis, but also, with varying prevalence, in other conditions. In SLE, these antibodies are neither sensitive nor specific for this condition. They occur, however, more frequently in (proliferative) lupus nephritis, particularly during active disease. Furthermore, levels of anti-C1q rise, in many cases, prior to a relapse of lupus nephritis, suggesting a pathogenic role for the autoantibodies. Indeed, experimental studies strongly support a pathogenic role for anti-C1q in immune complex-mediated renal disease. In addition, anti-C1q may interfere with the clearance of apoptotic cells, so influencing induction and expression of autoimmunity.