Membranous nephropathy was diagnosed in 54 patients between January 1975 and June 1983 in the Royal Infirmary, Glasgow. It was the commonest cause of the nephrotic syndrome and, with IgA nephropathy, the commonest primary glomerular disease. A cause was found in 10 patients. The last seven patients diagnosed were enrolled in the MRC trial. The natural history of the remaining 37 patients with idiopathic membranous nephropathy was studied. After an average observation period of 64 months, 50 per cent had stable renal function with or without proteinuria and 50 per cent had progressive renal failure or had died of other causes (five patients). Of the factors examined only heavy proteinuria and hypertension were significantly more common in patients who developed progressive renal failure. No patient who entered remission relapsed. Vascular complications were an important cause of morbidity and mortality. Incidence of events of arterial occlusion was significantly higher in these patients compared with patients with IgA nephropathy. Treatment of patients with membranous nephropathy should, therefore, be judged not only by its efficacy in preventing progressive renal failure, but also by its effect on vascular disease and by its toxicity.