Follow-up data on 100 patients with idiopathic membranous glomerulonephritis (IMGN), in whom renal biopsy has been performed, are discussed. The period of follow-up ranged from 3 to 24 years (median 7.8 years). Sixteen of the patients had end-stage renal disease. Complete remission of renal disease occurred in 41 patients. The remaining 43 patients exhibited persisting renal functional abnormalities at the end of the follow-up period. Using Cox's proportional hazard model, six parameters were found as predictive factors for prognosis at the time of biopsy: serum cholesterol level > 260 mg/dl, serum creatinine concentration > 1.4 mg/dl, the presence of tubulointerstitial (TI), vascular, mesangial, and Bowman's capsular lesions. No effect was observed in the size of subepithelial electron dense deposits (SED) on prognosis. A weakly positive correlation was found between the thickness of the glomerular basement membrane (GBM) and poor outcome. Treatment with steroid and/or immunosuppressive agents was not found to improve survival. Assessment of these factors thus allows the pathologist to make a relevant statement concerning the probable course and prognosis of the disease at the time of the diagnostic biopsy.