An extension of a previously reported quantitative electronmicroscopie study of the glomerular basement membrane in juvenile diabetes is presented. The initial phase in the development of basement membrane thickening in diabetic glomeruli has Mso been studied.-Measurements of the basement membrane were obtained from photomontages of glomerular cross sections produced from electron micrographs. A total of 16 glomeruli from five non-diabetics and 83 glomeruli from diabetics were measured.-The results showed that the peripheral glomerular basement membrane is normal at the onset of acute, juvenile diabetes, but a thickening is demonstrable in patients with a duration of the disease of about 2 years.-These findings support the hypothesis that diabetic angiopathy is a consequence of the metabolic derangement in diabetics.