Angiotensin-converting enzyme inhibitors (ACEI) can reduce proteinuria in diabetic and nondiabetic nephropathy. However, no studies have determined whether this antiproteinuric effect modifies the progression of renal insufficiency. We studied the evolution of 46 nondiabetic patients with nephrotic proteinuria treated with captopril for a minimum of 12 months. The follow-up period before captopril treatment was 12 to 18 months. At the end of follow-up, after captopril introduction (24.4 +/- 7.6 months), proteinuria had decreased from 6.3 +/- 2.5 to 3.9 +/- 3.1 g/24 h (P less than 0.001), with a mean decrease of 45% +/- 28%. The proteinuria decrease was higher in patients with reflux nephropathy, proteinuria associated with reduction of renal mass, inactive crescentic glomerulonephritis, nephroangiosclerosis, and IgA nephropathy, whereas patients with membranous glomerulonephritis and idiopathic focal glomerulosclerosis showed a poorer response. Patients were separated according to a proteinuria reduction greater (group A, 23 patients) or lower (group B, 23 patients) than 45% of the initial value. At the end of follow-up, renal function had not significantly changed in group A with respect to values at the start of treatment: serum creatinine (SCr) was 229 +/- 167 mumol/L (2.6 +/- 1.9 mg/dL) versus 203 +/- 97 mumol/L (2.3 +/- 1.1 mg/dL), and creatinine clearance (CrCl) was 0.80 +/- 0.52 mL/s (48 +/- 31 mL/min) versus 0.87 +/- 0.47 mL/s (52 +/- 28 mL/min). The slope of the reciprocal of Scr (1/SCr) showed a significantly beneficial change after captopril introduction.(ABSTRACT TRUNCATED AT 250 WORDS)