The aim of this study was to demonstrate the diagnostic value of changes in active renin concentration during the captopril test (measurements of plasmatic active renin concentrations, before 12.5 or 37.5 mg of captopril p.o., and 30 and 90 minutes after) in order to screen a significant renal artery stenosis. After a renal angiography, 88 hypertensive patients suspected of renovascular hypertension were classified according to the percentage of stenosis in the main renal artery: class I (< 30% - n = 50), II (30 to < 75% - n = 21), III (75 to < 90% - n = 8) and IV (90 to 100% - n = 11). The results of the captopril test were compared to those of renal angiography. The active renin before the test (basal AR), the greater increase in active renin after captopril (max AR), the difference between max AR and basal AR (DIF) and the active renin relative change after the test (RC) were compared in the 4 classes (ANOVA). There were no differences in diastolic blood pressure (> or = 90 mmHg) natriuresis (100 mmol/24 h in mean) between these different classes. The basal AR, the max AR, the DIF and the RC significantly differed between the 4 classes. They were greater in class III and IV than in class I. The positive criteria for the captopril test were max AR, DIF and RC. The positivity thresholds were max AR > or = 70 ng/l, DIF > or = 50 ng/l and RC > or = 165% (upper limit in the class I 95% confidence interval for each criterion).(ABSTRACT TRUNCATED AT 250 WORDS)