This is a retrospective study of 74 elderly patients (60 years of age or older) with idiopathic membranous nephropathy. They represented 23% of the total of 323 cases of idiopathic membranous nephropathy who presented during the 19-year review period. The mean age of these patients was 67 years versus 41 in the younger-onset group. The median presenting serum creatinine in the elderly group was higher (1.3 mg/dL v 1.0 mg/dL, P < 0.001), and the median creatinine clearance calculated by Cockcroft-Gault to correct for age, gender, and weight was lower (55 mL/min v 95 mL/min, P < 0.0001). The incidence of chronic renal insufficiency, defined as a creatinine clearance of less than 50 mL/min, was significantly worse in the elderly after a mean observation period of 47 months (59% v 25%, P < 0.0001) although end-stage renal failure (ESRF) was not (18% v 12%). The rate of change of renal function, however, as measured by the time to doubling of baseline creatinine, was similar in both groups, as was the complete remission rate. Forty-six percent of patients (33 of 74) in the elderly group received treatment: steroids alone (76%), immunosuppression drugs alone (9%), or a combination (15%). There was no benefit noted in terms of complete remission rate or incidence of chronic renal insufficiency. In summary, more elderly patients with membranous nephropathy develop chronic renal insufficiency, but this appears to be related to their age and decreased functional reserve, because the rate of decline in renal function after initiation of the disease is no different than in the younger age-group. The data also indicate that an accurate assessment of renal function in this older age-group requires an estimated or calculated creatinine clearance, given the inaccuracy when only serum creatinine is used. There was no evidence of improved outcome with prednisone therapy, and in view of the increased incidence of complications associated with this drug in the elderly as well as the decreased reserve at presentation, we suggest that routine steroid treatment of these patients should not be undertaken.