We retrospectively studied 82 consecutive patients with idiopathic membranous nephropathy accepted at two separate Renal Services. These patients had nephrotic syndrome at presentation and had been followed up for at least 10 years. The duration of the disease before renal biopsy ranged between 1 and 18 months. Forty-nine patients never received treatment (non-treated group) whereas 33 patients (treated group) received corticosteroids (and cytotoxic agents). The two groups were age and sex matched. There was no difference in clinical findings or in outcome between the two Renal Services. A complete and sustained remission of proteinuria was observed in about 25% of patients, and there was a significant difference (P less than 0.01) between the treated and non-treated group (mean value 39.1% vs 14.3%). Forty-four per cent of the non-treated patients and 24.2% of the treated ones developed chronic renal failure or renal death during a mean potential follow-up of 14 years. Non-renal death was recorded in 19.5% of the patients. The staging of the capillary wall lesion represented a very important prognostic index. We confirm, therefore, that idiopathic membranous nephropathy with nephrotic syndrome is frequently a progressive disease. Corticosteroids (and cytotoxic drugs) seem to be of some benefit in interfering with the natural course of the disease.