This study has confirmed impairment of the glomerular filtration rate (GFR) (increase of the serum creatinine concentration) by fibrosing processes in the renal cortical interstitium. In addition statistically significant correlations were found between the decrease of the total area of the proximal tubules and of the area of the epithelial cells and both the extent of the renal cortical interstitial fibrosis and the serum creatinine concentration. Further statistically significant positive correlations were observed between the age of the patients and both the grade of interstitial fibrosis and the serum creatinine concentration. No correlation could be established between the age of the patients and the total area of the epithelial cells of the proximal tubules. Pathogenetically it is conceivable that with progressive interstitial fibrosis the tubules become atrophic as a result of malnutrition. The function of these atrophied tubules may be disturbed, the reabsorptive capacity for NaCl impaired and consequently the GFR reduced not only by slowing of the glomerular blood flow secondary to interstitial fibrosis, but also by the tubular-glomerular feedback-mechanism.