To identify factors influencing the progression rate of chronic renal failure (CRF) in non-diabetics, the medical records of 124 patients with established CRF were reviewed retrospectively (serum creatinine between 200-350 mumols l-1 at the start of follow-up) for the period 1980-86. Sixteen patients were excluded because they did not fulfil the inclusion criteria. The remaining 108 patients were divided, using their original diagnosis, into four groups and were also assigned into one of three groups expressing their rate of progression. The definitions of these rates of progression were chosen arbitrarily. Comparisons with respect to mean arterial pressure (MAP), urinary protein excretion (UPE) and the progression rate of CRF were then made between the groups. In the group showing no progression of CRF (n = 17) there was a significantly lower MAP and UPE than in the two groups showing progression. Over all the patients and in the various diagnosis groups there was a strong correlation between UPE and the progression rate of CRF. There was no significant correlation between MAP and the progression rate of CRF in the diagnosis groups. These results indicate that an increasing magnitude of proteinuria is an adverse prognostic factor in CRF, irrespective of aetiology, and support the view that excessive glomerular protein filtration leads to glomerulosclerosis.