Renal biopsies from 16 patients with nephrosis, 7 patients with glomerulonephritis, and 3 patients with disseminated lupus erythematosus were studied with the electron microscope. The observations presented indicate that early in the course of each of these diseases alterations occur in the fine structure of the glomeruli which serve to distinguish one disease process from another. In nephrosis, some distortion of the organization of the epithelial foot processes was seen in all patients. These epithelial changes constituted the early, consistent lesion of the disease. There was frequently also a swelling of the endothelium. In glomerulonephritis, pronounced proliferative changes involving the endothelium and to a lesser extent the epithelium, together with the laying down of a basement membrane-like material, represented the predominate pathologic processes. There was also a swelling of both endothelial and epithelial cytoplasm. The epithelial foot processes generally appeared normal. In patients with a clinically" mixed" picture of nephrosis and nephritis, the glomerular changes were likewise" mixed," for various combinations of epithelial, endothelial, and basement membrane abnormalities were present. In disseminated lupus erythematosus, a more or less generalized thickening of the basement membrane proper associated with a variable degree of endothelial proliferation was seen. It is suggested that an accentuation of the process of basement membrane thickening results in the" wire loop" appearance sometimes seen by light microscopy. Although the earliest alterations in glomerular fine structure were characteristic for each of the disease processes, at later stages the changes were not always distinctive. The resulting scarred or" hyalinized" glomeruli, composed of relatively homogeneous, basement membrane-like material, and a few atrophic cells, appeared quite similar. Although the functional implications of the structural changes observed remain obscure at this time, it is believed that insight into mechanisms may stem from such observations.