The relationship between blood velocity and residual lumen diameter at the origin of 100 internal carotid arteries was studied. The diameter was measured by angiography and blood velocity by relatively simple Doppler ultrasound equipment. A scatter diagram between maximal velocity and diameter was used to construct a table on the sensitivity and specificity of various cutoff values for maximal velocity to diagnose stenoses of different degrees. A maximal velocity > or = 0.8 m/s had a sensitivity of 87% and a specificity of 93% to diagnose stenoses with a diameter < or = 3 mm (i.e. more than 50% diameter reduction). Thus, simple recording of maximal velocity can be used as a reliable screening method to detect patients with high- and medium-grade stenosis. Analysis of the spectral spreading of velocities across the lumen of a stenosis, or estimation of percent stenosis from the velocity in a stenosis and that in a poststenotic segment, were in our hands less useful for this purpose. None of the methods were good to detect or exclude the presence of low-grade stenosis.