Seven diabetic patients with membranous nephropathy were immunohistologically studied in order to clarify the role of extrinsic insulin in membranous nephropathy. In three cases (group A), granular deposits of insulin were detected along the glomerular capillary wall with indirect immunoperoxidase technique using anti-porcine insulin antibody, where IgG and C3 were deposited in the identical pattern. The other four cases (group B), 8 of idiopathic membranous nephropathy, and 5 of diabetic glomerulosclerosis showed no insulin deposit in the glomerulus. Clinically, proteinuria was heavier in group A (mean +/- SE; 32.0 +/- 5.4 g/day) than in group B (5.5 +/- 0.5). Nephrotic syndrome developed after the beginning of the therapy with porcine insulin, and in two of them, proteinuria was ameliorated after porcine insulin was replaced by human insulin. Since porcine insulin is a heterologous peptide for human beings and has antigenicity when injected into patients, immune complex composed of insulin and anti-insulin antibody may cause membranous nephropathy in some diabetic patients treated with this animal insulin.