From an electron-microscopic study in a case of diffuse membranous glomerulonephritis, we would like to propose a possible mechanism of hematuria in glomerular disease. There are several factors we should consider: (1) the deforming force of the red blood cell (RBC); (2) the deformability of the RBC; (3) the size of the gaps in the glomerular basement membrane (GBM), and (4) the thickness of the GBM. The deforming force is due to increased capillary pressure in the glomerulus that alters the contour of the RBCs. If these gaps are larger than 0.25 µm, the stretching and retracting force of the internal chamber of the GBM, combined with the capillary pulse, cause the deformed RBCs to pass through the gaps and proceed to the urinary space