In order to clarify the mechanism of proteinuria in diabetic nephropathy, ultrastructural changes of the glomerular basement membrane (GBM) in patients with diabetic nephropathy were examined by electron microscopy using our newly devised "tissue negative staining method". The normal human GBM showed a fine meshwork structure consisting of fibrils forming the small pores. The diameter of these pores was slightly smaller than that of human albumin molecules. The GBM in patients with diabetic nephropathy showed irregular thickening. At higher magnification, hitherto unknown cavities and tunnel structures, which were not seen in normal controls, were observed in the thickened GBM. In some portions, these cavities presented a honeycomb-like appearance. The diameters of the cavities and tunnels were far larger than the dimensions of albumin molecules. These enlarged structures are believed to allow serum protein molecules to pass through the GBM from the capillary lumen to the urinary space. These results suggest that the cause of massive proteinuria in diabetic nephropathy is the disruption of the size barrier of the GBM.