Seventy-two adults with the nephrotic syndrome without renal insufficiency had a membranous type of renal histology on biopsy. These patients were randomly allocated to at least eight weeks of alternate-day treatment with prednisone or placebo in a multicenter study. Deterioration of glomerular filtration rate was significantly more rapid in placebo-treated than in prednisone-treated patients, and ultimately 10 of 38 given placebo but only one of 34 given prednisone were in renal failure (creatinine more than 5 mg per deciliter [440 mumol per liter]) or dead (P less than 0.02). In male patients and in those with nonselective initial proteinuria, there was a trend (not reaching statistical significance) toward more rapid deterioration of renal function. Age, admission blood pressure, serum creatinine, daily total protein excretion, and severity of histologic changes did not predict the subsequent course. We conclude that a short course of alternate-day prednisone therapy was beneficial in our group of patients with idiopathic membranous nephropathy.