The clinical and histopathological features of 37 patients with idiopathic membranous nephropathy are presented. Males were four times as commonly affected as females and the age at presentation ranged from nine to 70 years. The period of observation varied from three months to 23 years. Twenty-eight patients (76 percent) presented with the nephrotic syndrome and nine patients (24 per cent) presented with non-nephrotic proteinuria. At the end of the study, of the patients presenting with the nephrotic syndrome, seven (25 per cent) were in remission, seven (25 per cent) remained nephrotic, nine (32 per cent) showed only proteinuria and five (18 per cent) were dead or on dialysis. Altogether eight patients (28 per cent) developed renal failure. The nine patients who presented with non-nephrotic proteinuria appeared to do better, and none developed renal failure. The occurrence of spontaneous remission makes assessment of benefit from immunosuppressive therapy difficult. However, analysis of our data and a review of the literature suggest that in this condition oral prednisone, cyclophosphamide and azathioprine have no significant therapeutic properties. Histological assessment confirmed the occurrence of mild (Grade 1) changes in patients biopsied soon after presentation, and tubular atrophy increased with the duration of illness. Immunofluorescence confirmed deposition of mainly IgG and complement. Repeat biopsies in 14 patients showed no histological improvement and remission was not accompanied by resolution of histological abnormalities.