Anti-C1q antibodies were first identified as low-molecular weight C1q precipitins in the sera of patients with systemic lupus erythematosus (SLE) over 30 years ago. Anti-C1q antibodies are strongly associated with the development of proliferative lupus nephritis, so much so that active renal inflammation in SLE patients is very unlikely if these antibodies are not present. These antibodies seem to develop through mechanisms that are similar to those of other SLE autoantibodies.