End diastolic flow velocity just beneath the aortic isthmus assessed by pulsed Doppler echocardiography: a new predictor of the aortic regurgitant fraction. End diastolic flow velocity just beneath the aortic isthmus was measured within 72 hours of cardiac catheterisation by pulsed Doppler echocardiography in 30 controls and 61 patients with aortic regurgitation. The end diastolic flow velocity was determined at the peak R wave on a simultaneously recorded electrocardiogram. In all controls there was no reverse flow at the end diastole beneath the aortic isthmus. In patients with aortic regurgitation the end diastolic flow velocity correlated well with the angiographic grade of regurgitation (r = 0.81) and regurgitant fraction (r = 0.82). The mean (SD) values were 6.3 (5.2), 12.2 (4.3), 22.1 (5.7), and 34.3 (9.3) cm/s for patients with regurgitant fraction of less than 20%, between 20% and 40%, between 41% and 60%, and greater than 60%, respectively. An end diastolic flow velocity of greater than 18 cm/s predicted a regurgitant fraction of greater than or equal to 40% with a sensitivity of 88.5% and a specificity of 96%. The study suggests that the pulsed Doppler derived end diastolic flow velocity is a useful index in the routine non-invasive assessment of the severity of aortic regurgitation. 