In a retrospective study, the clinical course of 23 adult patients with documented idiopathic membranous nephropathy was reviewed. These patients included 16 men and 7 women, who were followed for a period of as long as 9 years. 22 patients (95.7%) were nephrotic (proteinuria of 3 g or more in 24 h) and 6 (26%) were hypertensive. 5 patients were treated with prednisone alone, 12 with prednisone and cyclophosphamide, 2 with prednisone and azathioprine and 4 received no treatment. 18 patients achieved complete remission (78.3%). 16 of these (88.9%) were treated with prednisone, or prednisone and cyclophosphamide or azathioprine, and the remaining remitted spontaneously. In 3 additional patients, proteinuria improved with treatment with prednisone and immunosuppression but remained in the nephrotic range in 2, and fell to 1.8 g/day in 1. Patients who failed to respond to steroids showed remarkable improvement when therapy with cyclophosphamide was added. Of the 4 patients not treated, 2 underwent spontaneous remission and 2 developed renal insufficiency. These results suggest a beneficial effect of steroids in the treatment of membranous nephropathy, and suggests further that cyclophosphamide may be effective in patients not responding to steroids alone. Finally, these results suggest that in many patients therapy may have to be prolonged before a remission can be observed.