The clinical course of 139 patients (77 male, 62 female) with idiopathic membranous glomerulonephritis is reviewed. The median duration of follow-up was 52 months; 45% and 25% were followed for more than 5 and 10 years respectively. The median age at presentation was 36. Fifty-four percent of patients had the nephrotic syndrome at presentation. Half of the patients were treated at some stage with cyclophosphamide or corticosteroids. During the course of follow-up some deterioration in renal function occurred in only 20% of patients. The patients who suffered deterioration in renal function were mainly male and had significantly worse renal function and a higher incidence of the nephrotic syndrome than the other patients at presentation. Only 7 male patients (5%) developed terminal renal failure during follow-up and one female presented in terminal renal failure. Survival was 88% and 81% at 5 and 10 years. The median predicted (or actual) time for development of terminal renal failure in patients with progressive deterioration was 7.3 years. These data are in accord with other recently published series which have described a relatively benign prognosis for idiopathic membranous glomerulonephritis.