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 less than 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.