Sixty-seven adults with idiopathic membranous nephropathy and the nephrotic syndrome were randomly assigned to symptomatic treatment only or to a six-month course of methylprednisolone alternated with chlorambucil every other month. Patients were followed for one to seven years. At the end of follow-up (mean of 31.4 +/- 18.2 months for the treated group and 37.0 +/- 22.0 for the control group) 23 of 32 treated patients were in complete or partial remission, as compared with 9 of 30 control patients (P = 0.001). Twelve of the treated patients were in complete remission, as compared with only two of the controls. In the treated group there were no changes in renal function during follow-up, whereas in the control group the reciprocal of the plasma creatinin level, which is proportional to the creatinine clearance, decreased significantly (P = 0.00017) after two years of follow-up. Side effects were minimal in all treated patients except two, who were dropped from the study because of peptic ulcer and gastric intolerance to chlorambucil. We conclude that steroid and chlorambucil treatment for six months favors remission of the nephrotic syndrome in adults with idiopathic membranous nephropathy and can preserve renal function for at least some years.