To clarify the association between certain immunologic factors and clinical activity in patients with systemic lupus erythematosus, 96 patients were studied. Those with antibodies to deoxyribonucleic acid (DNA) or heat-denatured DNA, or with serum complement levels of less than 50 C′H50 units per ml, were more likely to have renal involvement. Very low complement levels and high titers of complement-fixing antibodies to DNA were always associated with active disease, especially active renal disease, whereas the absence of these abnormalities usually indicated inactive renal disease. A 50 per cent fall in serum complement levels in 22 patients was accompanied by, or preceded the onset of, active nephritis in 19 patients. These serologic factors may thus reflect the in vivo formation of immune complexes that cause nephritis. Serial immunochemical observations may be useful in the management of patients with systemic lupus erythematosus.