In a retrospective study of the effect of treatment in biopsy-proved idiopathic membranous nephropathy, 91 adults and 12 children were followed for periods up to 29 years after clinical onset (mean, 6.5 years). Forty-four were treated with corticosteroids, 15 with corticosteroids and immunosuppressants; 44 had no treatment and served as a control group. Clinical cure and improvement were significantly greater in the treated than in the nontreated group (P<0.01). The recurrence rate, occurrence of renal failure and probability of death were significantly greater in the nontreated group, although some of these patients eventually showed improvement. Prognosis was better in patients who responded to therapy. These data strongly suggest that steroid therapy is beneficial in patients with membranous nephropathy. (N Engl J Med 295:741-746, 1976). Membranous nephropathy is a glomerular disease manifested by proteinuria and well defined lesions of the capillary wall. Some patients with these lesions have other, associated systemic diseases such as lupus erythematosus or diabetes mellitus. In others the glomerular lesions may be related to drugs, chemicals or specific infections. In many such cases clinical remission may follow cessation of exposure to the specific agent. The majority of patients with membranous nephropathy do not have any known predisposing or associated cause and constitute the idiopathic group. The natural course of the idiopathic form is often characterized by slow clinical progression or episodes.