Few studies describe the treatment of membranous nephropathy associated with systemic lupus erythematosus. Although cyclosporine-A has been used to treat patients with the nephrotic syndrome and also with systemic lupus, only a few of these patients have had lupus membranous nephropathy. In this pilot study, we assessed the safety and efficacy of cyclosporine in ten nephrotic patients with either pure membranous lupus nephropathy (seven patients) or membranous lupus nephropathy with superimposed mild proliferative lesions (three patients). Cyclosporine (4-6 mg/kg/day) alone (2 patients), or in conjunction with low dose corticosteroids (8 patients) was given for a period of up to 43 months. Six patients achieved a nadir proteinuria of less than 1 gram daily, two patients decreased urinary protein excretion to 1-2 grams daily, and the remaining two patients continued to excrete over 2 grams of protein daily. All patients experienced symptomatic improvement of their nephrotic syndrome and serum creatinine was not significantly increased at the end of the study period. Three patients with superimposed mild proliferative lesions experienced renal and systemic lupus flares while on treatment requiring additional immunosuppressive therapy. Side-effects were minor except for transient rises in serum creatinine in one patient and a case of drug-related hepatitis possibly caused by cyclosporine. Repeat renal biopsies in five patients revealed a decrease in the lupus activity index and a rise in the chronicity index. There was an increase in the stage of the membranous nephropathy on these repeat biopsies, but a reduction in the number of fresh deposits.(ABSTRACT TRUNCATED AT 250 WORDS)