The evaluation of renal biopsies from diabetic patients rests upon measurements of many structures in glomeruli, the tubules and the interstitium. These individual values can also be expressed as an index of renal structure. To assess the validity of the individual values versus a published index of renal structure, we evaluated glomerular basement membrane width, mesangial volume fraction, other measures of glomerular structure, and percentage glomerulosclerosis in type 1 diabetic subjects with renal function ranging from normal to marked albuminuria, hypertension, and a falling glomerular filtration rate. For all measured structural parameters and for the index, all groups of diabetic patients, even those who had completely normal renal function, had values different from non-diabetic subjects (P<0.001). Furthermore, there was an excellent correlation between the structural index and a similar functional index (r=0.77).