The objective of this trial was to determine the long-term antiproteinuric effect of a low-dose of the angiotensin-converting-enzyme inhibitor (ACEI) captopril in patients with idiopathic membranous nephropathy (IMN), stable renal function, and no indicators of poor long-term prognosis. Fourteen adult IMN outpatients (median age 53 years) with a median duration of disease of 3 years received 12.5 mg of captopril twice a day for 12 months in a prospective trial. The effects of therapy were evaluated on the basis of plasma creatinine, 24-h proteinuria, the proteinuria selectivity index, albuminaemia, and serum IgG levels. Data were compared by means of the non-parametric paired Wilcoxon test. Three patients withdrew from the trial. Renal function remained stable in the 11 who completed the study. A clear decrease in proteinuria was observed after 1 month of therapy, which persisted with time and was associated with a trend towards a further long-term decrease. An increase in serum albumin was only observed after 6 months of therapy, again with a trend towards an increase over time. Serum IgG levels increased during therapy. This study together with data from the literature suggests a potential long-term benefit of angiotensin-converting-enzyme inhibitors in moderately proteinuric IMN. Prospective trials comparing low-dose and high-dose ACEI to no treatment or a placebo in non-severe IMN are now required.