Sixty-four patients (47 male, 17 females) aged between 20 and 70 years with idiopathic membranous glomerulonephritis, never having received steroid or immunosuppressive drugs prior to and subsequent to biopsy, have been followed to terminal renal failure or for a minimum of two up to fifteen years. Presentation was with asymptomatic proteinuria in 12 and nephrotic syndrome in 52. The serum creatinine at time of biopsy was less than or equal to 99 mumoles/l in 25, 100-119 mumoles/l in 18 and greater than or equal to 120 mumoles/l in 21. During the follow up there was no deterioration in renal function in 30 patients (48%). In 27 patients (43.5%) there was a steady deterioration in renal function, on the average 30 months (range 5-60) for the serum creatinine to double and 32 months (range 5-49) for the serum creatinine to reach 400 mumoles/l. In five patients there was a slow deterioration. A plot of the reciprocal of the sequential serum creatinine values with time indicates that the rate of deterioration is essentially constant in any patient but that there is a wide variation between patients. The reciprocal of the serum creatinine is a useful means of following the evolution of the disease. In two patients there was a change in the rate of deterioration and a cause could be identified (one with renal vein thrombosis, one with interstitial nephritis). Poor prognostic indicators were: nephrotic syndrome at presentation, impaired function at time of diagnosis, male patients and older age.