The peak pressure drop across the aortic valve in aortic stenosis has been measured by Doppler ultrasound. Maximum velocity in the Doppler signal from the aortic jet was recorded using a maximum frequency estimator. With an angle close to zero between ultrasound beam and maximal velocity in the jet, peak pressure drop can be calculated from the maximal velocity measured; a larger angle will underestimate maximal velocity and pressure drop. In 57 of 63 patients with aortic stenosis, the aortic jet could be reached by the ultrasound beam and, in 37 of these, peak pressure drop by ultrasound was compared with that obtained at catheterisation. In patients less than 50 years of age the aortic jet was easy to find, the measurement was reproducible, and underestimation of the pressure drop obtained at catheterisation was within 25 per cent in 17 of 18 patients. In patients over 50 years Doppler signals from the aortic jet were more difficult to obtain, and pressure drop was significantly underestimated in one-third, but time of maximum velocity in systole could indicate whether moderate or severe aortic stenosis was present.