The volume of the glomerular tuft and its components was estimated in juvenile diabetics at onset and after one to six year's duration of the disease. An enlargement of the glomerular tuft was demonstrated in the newly diagnosed diabetics. In these patients the volume of both the individual capillary lumina and of the individual glomerular cells was enlarged, whereas the number of cells was unchanged. Although a partial normalization is seen in diabetics after a few years' disease, such patients still have an elevated capillary volume. The possible relationship between these findings and the well-documented elevation of the glomerular filtration rate in diabetics is discussed. It is suggested that the mechanism behind the morphological and functional changes is either a pressure-induced unfolding of the capillary wall or an increase of the capillary wall area, but the present study permits no distinction between these possibilities.