The present electron microscopic study shows that the kidney hyperfunction in early diabetes can be due to a significant morphological change: an increased glomerular filtration surface. Applying standard stereological methods, the area of the peripheral wall of the glomerular capillaries was measured in biopsy specimens obtained from 7 patients with early diabetes and 7 controls.--An 80% enlargement of the capillary wall (the surface of the peripheral basement membrane) was found in the diabetics (2p=0.0096). Also the total area of the interface between the tuft and the urinary space was increased by 70% (2p=0.029). Since the thickness of the peripheral basement membrane is known to be unchanged in patients with early diabetes the finding of an increased area of the membrane implies that an increased quantity of basement membrane material is present in these patients. The significance of this phenomenon for the understanding of the metabolism of the basement membrane is discussed, and a working hypothesis is advanced for the pathogenesis of the diabetic microangiopathy.