Morphometric investigations were performed in 18 biopsies from 16 patients with the nephrotic syndrome and 10 biopsies from 10 patients with mild proteinuria not associated with oedema. All biopsies showed normal glomeruli on light microscopy. The interstitial fibrosis was significantly increased in both patient groups compared with controls, and it was greater in the patients with mild proteinuria than in the group with nephrotic syndrome. A significant negative correlation was present between the degree of fibrosis and renal function in both groups of patients.