We developed an approach in quantifying the risk of developing chronic renal insufficiency (CRI) based on a cohort of 184 patients with idiopathic membranous glomerulonephritis (IMGN), prospectively followed by the Toronto Glomerulonephritis Registry between 1974 and 1988. After a mean follow-up period of 5.8 years, 26% of patients developed CRI (defined as persistent reduction of creatinine clearance (CCr) less than or equal to 60 ml/min/1.73 m2 for greater than or equal to 12 months). We found that when compared to the baseline probability of the unselected patients, the severity of proteinuria at kidney biopsy added only marginally to the prediction of CRI. We introduced a special test condition: persistent proteinuria (PP) (that is, duration of proteinuria, g/day, above different cut-off levels). We examined the positive predictive value (PPV) and sensitivity (SEN) of 15 arbitrarily chosen levels of PP (that is, proteinuria greater than or equal to 4, 6 or 8 g/day persisting for greater than or equal to 6, 9, 12, 18 or 24 months) to select levels with optimal predictive characteristics. We found that PP greater than or equal to 8 g/day for greater than or equal to six months was a simple and useful predictor of CRI with a PPV and SEN of 66%. To further improve our prediction, we tested the following parameters: age, sex, initial SCr and CCr, proteinuria, serum albumin, hypertension, rate of change of CCr over time, and therapy (steroids +/- immunosuppressive drugs) in a multivariate analysis. Proteinuria, initial CCr, and rate of change of CCr were most important in predicting CRI.(ABSTRACT TRUNCATED AT 250 WORDS)