Defining the most appropriate treatment for patients with idiopathic membranous nephropathy is a matter of controversy. The course of the disorder is often benign, and the immunosuppressive regimens used in some patients have uncertain benefits and substantial risks. We studied the natural history of idiopathic membranous nephropathy in patients who received only symptomatic therapy.We prospectively studied 100 consecutive patients (68 men and 32 women; mean [+/- SD] age, 51 +/- 17 years) with biopsy-proved idiopathic membranous nephropathy. The patients received diuretic or antihypertensive drugs as needed, but no glucocorticoid or immunosuppressive drugs. We examined the patients and measured their urinary protein excretion and serum creatinine concentrations every 6 months for a mean of 52 months.Twenty-four (65 percent) of the 37 patients followed for at least five years had complete or partial remission of proteinuria; in 6 others (16 percent), end-stage renal disease developed, and they required dialysis. As calculated by the Kaplan-Meier method, the estimated probability (+/- the standard error of the estimate) of retaining adequate kidney function was 88 +/- 5 percent after five years and 73 +/- 7 percent after eight years. The prognosis was poorer in men and in patients over 50 years of age, but not in patients with the nephrotic syndrome, hypertension, or hypercholesterolemia.Most untreated patients with idiopathic membranous nephropathy maintain renal function for prolonged periods and are likely to have spontaneous remission. These results do not support the use of glucocorticoids and immunosuppressive drugs in patients with idiopathic membranous nephropathy.