To evaluate ethnic differences in cerebral circulation, an index of carotid vascular resistance (RI) was determined by continuous wave Doppler ultrasound. Validation of the index resulted from a significant correlation between RI and carotid vascular resistance obtained from direct measurement of blood pressure and carotid blood flow by means of range-gated pulsed Doppler (r = 0.64; P less than 0.01). We studied 37 normotensive men: 17 white and 20 black. The two populations were age, sex, body mass index and mean arterial pressure matched. In blacks, a significant higher RI was observed (74.10(-2) +/- 1.6.10(-2) versus 64.10(-2) +/- 1.9.10(-2); P less than 0.001) together with a significantly higher pulse pressure (51 +/- 3 mmHg versus 43 +/- 2 mmHg) in comparison with whites. Our findings suggest that the higher prevalence of stroke in blacks might be related to significant haemodynamic characteristics of the carotid-cerebral circulation involving cerebral vasoconstriction.