An electron-microscopic (EM) study of the non-sclerotic glomeruli was made in 96 children with the nephrotic syndrome in whom light microscopy had shown minimal change, focal global glomerulosclerosis or segmental glomerulosclerosis. EM showed a variety of alterations. Foot process fusion, duplication and crenation of the lamina densa, and granular and lucent expansion of lamina rara interna were noted in almost all patients in all three groups. Localised ulceration of the podocytes was noted in some patients in each group. There was no difference in the mean thickness of the glomerular basal lamina but there was an increase with age in minimal change. Curious extracellular curved striated bodies, clusters of electron-dense, round microparticles and microfilaments were found in all three, but most frequently in segmental glomerular sclerosis. Electron-dense deposits were seen in all but one of the patients with segmental glomerular sclerosis and usually involved the capillary wall. They were seen in one third of those with minimal change and focal glomerular sclerosis but rarely in the capillary wall. There were no specific features which distinguished segmental glomerular sclerosis although the various types of deposits were more extensive and more frequent than in minimal change and focal glomerular sclerosis. These observations are consistent with common pathogenetic factors operating at different intensities in segmental glomerular sclerosis, focal glomerular sclerosis and minimal change.