The ultrastruetural changes in the kidneys of 42 children suffering from nephrotie syndrome were examined with eleetronmieroseope. The cause of the nephrotie syndrome was observed to be in 85O/o a proliferative glomerulonephritis and only rarely a membranous glomerulonephritis. In both forms amorphous deposits of different sizes, loealisation and electron density were found. With the aid of these observations a classification of the nephrotic syndrome into the corresponding inflammatory stages could be made. The ultrastruetural changes of the glomerulus were caused on the one hand by the inflammatory process and on the other hand by renM hypertension and proteinuria.