Twenty renal biopsy specimens from patients with idiopathic diffuse membranous glomerulonephritis (MGN), for whom both light and electron microscopy as well as immunofluorescence microscopy and full clinical data were available, were examined quantitatively and compared with fifteen cases of normal controls. Morphometric investigations were performed by means of a computer image analysis system to evaluate whether morphometric analysis may be helpful in morphological diagnosis of MGN, and to study whether serum creatinine and changes in quantitatively analysed glomeruli could correlate with the interstitial fibrosis. The study revealed that total glomerular area, total number of glomerular cells per total glomerular area, and total glomerular cells per unit of glomerular area as well as relative interstitial volume were significantly increased in MGN in comparison with normal glomeruli. Moreover, there was significant positive correlation between the width of the cortical interstitium and serum creatinine level but not between the width of the cortical interstitium and glomerular changes. These findings suggest, that in MGN mainly interstitial lesions are involved in the renal prognosis and they are not related to the severity of glomerular changes. The authors conclude, that although morphometric methods supported by image analysis system cannot be used instead of light microscopy, electron microscopy and immunohistochemistry, they may be helpful in every-day diagnostic procedures of early stages of MGN.