Thirty four patients with aortic regurgitation were studied by continuous wave Doppler ultrasound. In 30 of these the regurgitation was graded by cineangiography as mild, moderate, or severe and in four severe regurgitation was confirmed at operation. The half times of the aortoventricular pressure differences obtained with Doppler compared well with those obtained from pressure recordings at catheterisation. The relation between pressure half times and cineangiographic gradings of severity was not consistent. Similarly, a control group of patients without aortic regurgitation showed a wide range of the invasively recorded pressure half times which overlapped with those in patients with aortic regurgitation. This suggests that factors such as systemic vascular resistance, and aortic and left ventricular compliance can have an appreciable effect on the pressure half time. If these factors are not included the method will be of limited value, except in patients with pressure half times of less than 300 ms, when regurgitation invariably is severe. These results suggest that at present the method is of value only in recognising the patients with the most severe aortic regurgitation who need early operation.