We conducted a controlled trial to investigate the long-term effects of treatment with methylprednisolone and chlorambucil in patients with idiopathic membranous nephropathy. We have previously reported that after a mean of 31 months, treated patients did better. We now report the results of a longer follow-up. Eighty-one patients with proteinuria (greater than or equal to 3.5 g per day) and biopsy-proved membranous nephropathy were randomly assigned to receive either supportive therapy alone or a six-month course of corticosteroids alternated with chlorambucil (0.2 mg per kilogram of body weight per day) every other month. Methylprednisolone was first given intravenously in three pulses (1 g per day) and was then given orally (0.4 mg per kilogram per day) for 27 days. The patients were followed for 2 to 11 years (median, 5). Two patients in the control group and one in the treatment group died. At the last follow-up visit, 9 of 39 patients assigned to the control group (23 percent) and 28 of 42 patients assigned to the treatment group (67 percent) did not have the nephrotic syndrome. At five years there were more remissions of the nephrotic syndrome in treated patients than in controls (22 of 30 vs. 10 of 25; P = 0.026). Compared with base-line values, the mean reciprocal of the plasma creatinine level declined significantly in the control group (33 percent; P = 0.0002) but not in the treatment group (6 percent; P not significant). Plasma creatinine increased by 50 percent or more in 19 controls (49 percent) and in 4 treated patients (10 percent). We conclude that a six-month course of methylprednisolone and chlorambucil can bring about sustained remission of the nephrotic syndrome and help to preserve renal function in patients with idiopathic membranous nephropathy.