In 45 patients with essential hypertension and 15 age-matched normotensive control subjects, the renal resistive index, as an expression of arterial impedance, was determined using Doppler ultrasound. In both kidneys the resistive index was assessed at baseline and after captopril test (50 mg orally). In the moderate and severe hypertensives, compared to mild hypertensives and control subjects, the baseline resistive index was significantly higher (P less than .05). Following captopril, the resistive index increased only in normotensives (P less than .05) and in mild hypertensives (P less than .05). Univariate and multivariate analyses show that the duration and severity of hypertension correlated with an increase of the resistive index both in basal and in dynamic conditions. Thus, the use of the resistive index, as determined by echo-Doppler, could provide useful information for the assessment of renal vascular impedance in essential hypertensive patients. This would help us detect the evolution of hypertensive disease to the higher degrees of severity that are correlated to renal arteriolar damage.