Electron microscopic examination of percutaneous renal biopsy specimens from 12 normotensive patients ranging in age from 13 to 23 years with diabetes of juvenile onset revealed that glomerular alteration is not an invariable manifestation of early stages of diabetes and that changes in glomerular basement membrane most likely result from rather than precede those of the mesangium. Arteriolar hyalinosis was the most frequent alteration observed in these patients, occurring in 75% (9) of the specimens. This abnormally high prevalence of arteriolar hyalinosis is in accord with the recognized tendency of vascular disease to develop in patients with diabetes. The ultrastructural appearance of the renal arteriolar hyalinosis in our patients was indistinguishable from that in patients with essential and renal hyptertension and that in some normotensive older adults. These observations provide some information unifying the triad of hypertension, aging, and diabetes mellitus.