We studied the long-term effect of an angiotensin converting enzyme (ACE) inhibitor, captopril, on the progression of chronic renal failure and on the rate of urinary protein excretion. When compared with standard triple therapy, captopril slowed the progression of renal failure. Captopril was also able to reduce the proteinuria of non-diabetic glomerular origin. This reduction was not dependent on the presence or absence of arterial hypertension but was limited by the presence of low serum albumin levels, and only occurred in patients with proteinuria in excess of 3 g/24 h.