A retrospective long-term study (average follow-up time 5.2 years) of 334 patients with idiopathic membranous glomerulonephritis (MGN) was carried out with the following results: 1) MGN was found to have a relatively good prognosis when all cases were considered together: 5-year kidney survival rate (KSR) -88%, and 10-year KSR -77%. 2) Univariate survivorship analysis showed the following morphological and clinical parameters to be associated with an increased risk of terminal renal insufficiency or death from renal disease: a) tubulo-interstitial changes; b) glomerular stage III as opposed to stages I and II; c) elevation of serum creatinine concentration at the time of the biopsy; d) arterial hypertension at the time of the biopsy. 3) Multivariate analysis showed that only tubulo-interstitial changes (interstitial fibrosis and/or acute renal failure) found at the time of the biopsy and their clinical correlate, serum creatinine concentration, were significant and therefore of definite prognostic importance. 4) Unsystematic therapy with steroids and/or cytostatic agents does not improve the long-term prognosis of MGN. 5) The cause of disease in the tubulo-interstitial system in MGN is discussed. Interstitial fibrosis is considered to develop possibly as a consequence of unresorbed interstitial edema which can develop during an episode of acute renal failure. Coexisting T-cell-mediated disease in the region of the intertubular capillaries is also considered as a possible factor in the development of interstitial fibrosis.