By using histologic, immunofluorescent, and electron microscopic observations of 52 patients with systemic lupus erythematosus and renal involvement, three different forms of lupus nephritis, each having typical clinical features and course, were identified. Focal proliferative lupus nephritis was characterized by urinary abnormalities, rarely with the nephrotic syndrome or renal insufficiency; the renal manifestations remitted readily in association with corticoid therapy, and progression to renal failure did not occur. Diffuse proliferative lupus nephritis was characterized by the nephrotic syndrome, usually with renal insufficiency; remissions were observed in about half of the patients, most of whom relapsed, and most patients with the diffuse proliferative type died of renal insufficiency within 3 years. Membranous lupus nephritis was characterized by the nephrotic syndrome; remissions were observed in about one third and were occasionally sustained—in most, the nephrotic syndrome was persistent, and renal insufficiency developed. Marked diastolic hypertension, papilledema, heart failure, encephalopathy, and rapidly progressive renal failure was observed in five patients in whom the kidney at postmortem showed fibrinoid arteritis and arteriolitis.