The mean pressure difference across the valve in aortic stenosis is an indicator of the severity of the obstruction to flow. Non-invasive determination of the mean pressure gradient by Doppler ultrasonography is, however, complicated by the squared relation between instantaneous velocities and pressure differences. The validity of a new simple formula for calculation of the mean pressure difference from the peak pressure difference was evaluated in 26 patients with aortic stenosis. The formula is: delta pmean = 0.64 delta ppeak, where delta pmean is the mean pressure gradient and delta ppeak the peak pressure gradient. There was a close correlation between the mean pressure differences determined by application of the formula to the peak pressure differences measured at catheterisation and the mean pressure differences obtained by planimetry (r = 0.97, SEE = 4.7 mm Hg). The correlation between mean pressure differences determined by continuous wave Doppler ultrasound and the formula and those measured by planimetry was also close (r = 0.91, SEE = 7.6 mm Hg) and only three patients showed a difference between the two methods of greater than 10 mm Hg. The new formula is a simple and reliable means of estimating the mean pressure difference from Doppler recordings and it facilitates the comparison of Doppler and catheterisation data.