Correlations between the relative volume of the intertubular capillaries in the renal cortex and the serum creatinine concentration in primary glomerulopathies, renal vasculopathies, and chronic interstitial nephritides are reported. In the mesangioproliferative glomerulonephritides, there are significant negative correlations between the number and area of the intertubular capillaries in the cortex and the serum creatinine concentration. In diabetic glomerulosclerosis, renal glomerular amyloidosis, decompensated benign nephrosclerosis, secondary malignant nephrosclerosis, and chronic interstitial nephritis, there is a significant negative correlation between the relative area of the intertubular capillaries and the serum creatinine concentration. Thus, in these diseases, there is progressive narrowing/ obliteration of the postglomerular capillaries which leads to a progressive decrease in glomerular filtration rate and thus to a rise in serum creatinine concentration.