One hundred and four consecutive patients with idiopathic membranous nephropathy have been followed for 24 years. The actuarial survival rates in 5, 10 and 15 years were 94.6, 90.0 and 80.3%, respectively. Within the first 5 years clinical status improved gradually, but thereafter entered into an equilibrated state, when approximately 70% of the patients were in remission of absent (40%) or less than 1.0 g/24 hour urinary protein. As for renal function, within the initial 5 years around 80% of the patients revealed a glomerular filtration rate (GFR) greater than 80 ml/min. This percentage reduced approximately to 70 and 50% over periods of 10 and 15 years, whereas a mean reduction rate of GFR was only 15 ml/min over a period of 8 years. The clinical status at the 5th year correlated well with its outcome (rs = 0.676, p less than 0.001) and GFR estimated around the 5th year with the final estimate (r = 0.796, p less than 0.001). These data suggest that membranous nephropathy follows a fairly good clinical course, and that the 5th year clinical status and renal function are valuable prognostic indices of the disease.