Membranous nephropathy is a relatively common glomerular disease found to underlie both nonnephrotic and nephrotic proteinuria. In adults, about 75% of cases are primary (idiopathic) and 25% are secondary to a wide variety of causes, including neoplasia, infections, autoimmunity, and drugs. Presenting features are not distinctive enough to permit a diagnosis without a renal biopsy examination. Serologic studies are normal in the idiopathic disorder. The morphologic features are characteristic and include gradual thickening of the capillary wall caused by the in situ deposition of immune complexes accompanied by new basement membrane synthesis. The natural history of the untreated disorder is variable. Spontaneous remissions (complete and partial) of proteinuria, usually accompanied by stable renal function, eventually occur in 40% to 50% of patients and the remainder slowly progress to end-stage renal disease (ESRD) or die of complications or from unrelated disease after 5 to 15 years. Factors associated with a progressive course include older age at onset, male gender, persisting hypertension, hyperlipidemia and/or hypoalbuminemia, reduced renal function at discovery, persisting nephrotic range glomerular proteinuria, concomitant tubular proteinuria, and advanced glomerular damage with chronic tubulointerstitial fibrosis.