Twenty-eight patients with SLE and distinct, well-defined renal morphologic lesions of membranous nephropathy were followed up for 4 years. These patients comprised approximately 8% of the patients evaluated for SLE during a 12-year period. The patients with membranous lupus nephropathy had typical systemic features of SLE, and most of them had positive LE cell tests and ANA, low serum complement concentrations, and mildly elevated serum antinative DNA levels. Proteniuria and microscopic hematuria were usually discovered years after systemic symptoms of SLE had developed, Only two patients had slowly progressive renal failure, and most patients continued to have proteinuria. Prednisone treatment did not influence either proteinuria or renal function. In only one patient, the renal character of the disease changed drastically, demonstrating membranoproliferative glomerulonephritis. Six patients died (21%); most of these died of cardiovascular illnesses. The relatively benign and stable renal course of membranous lupus nephropathy in patients with otherwise typical SLE suggests that the renal pathogenesis is different from that of proliferative lupus nephritis.