Duplex Doppler ultrasonography has been validated as a noninvasive method to evaluate hemodynamic features of renal blood flow in renal and intrarenal arteries in patients with various renal diseases. The significance of duplex Doppler sonography in the evaluation of renal vascular resistance in essential hypertension has not yet been clearly determined. The aim of the present study was to evaluate the renal vascular resistance in patients with essential hypertension by measuring intrarenal arterial resistance (RI) and to correlate RI with renal functional tests and other clinical and laboratory data. Duplex Doppler ultrasonography was used to measure RIs in intrarenal arteries in 128 patients with essential hypertension and 61 age-matched normotensive control examinees. The renal vascular resistance index (RI) was determined by use of Doppler ultrasound. Hypertension was classified according to the 1997 Joint National Committee Guidelines (JNC-VI). Mean RI in hypertensive patients was 0.66 +/- 0.05 (+/- sd) and in healthy controls 0.60 +/- 0.03 (+/- sd) (p = 0.0001). RI correlated significantly with patient's age (r = 0.577, p = 0.001), duration of hypertension (r = 0.335, p= 0.001), stage of hypertension according to the JNC-VI classification (r = 0.315, p = 0.006), creatinine clearance (r = -0.383, p = 0.001), systolic blood pressure (SBP, r = 0.41, p = 0.001) and mean blood pressure (MBP, r = 0.30, p = 0.002). RI values did not correlate significantly with plasma renin concentration (r = -0.198 NS), diastolic blood pressure (DBP, r = 0.17, p = 0.06), and cardiac pulse (r = -0.10, p = 0.16). Multiple regression analysis showed that independent variables for RI were the patient's age (multiple R = 0.53, signif. F = 0.001) and systolic blood pressure (multiple R = 0.57, signif. F = 0.03). The renal Doppler resistance index (RI) is increased in essential hypertension and it correlates with renal functional tests as well as with patient's age, duration of hypertension, with a stage of hypertension according to the JNC-VI classification, and with systolic and mean blood pressure. The increased renal vascular resistance (RI) in hypertensive patients could be a sign of developing hypertensive nephrosclerosis and consequently renal failure. The utilization of the renal vascular resistance index (RI), provides a new noninvasive parameter in the followup of patients with essential hypertension.