Left atrial pressure (LAP) can be measured from the diastolic blood pressure and mitral regurgitation (MR) Doppler spectrum, but this method is cumbersome and requires meticulous evaluation of hard-copy recordings. We developed and evaluated custom software which processes digital MR Doppler signals for use in measurement of LAP. Methods: Digital MR and aortic valve Doppler spectra were recorded in 14 patients during pulmonary artery catheterization. The interval from the ECG R-wave to aortic valve opening (AVO) was determined from the aortic valve Doppler. MR spectra were processed by averaging grey levels of individual ECG-gated pixel coordinates from 5-10 beats. The signal-averaged MR spectrum was used to determine the LV-LA gradient at AVO, which was subtracted from aortic diastolic pressure to give LAP. Results: Signal averaging (right picture) enhanced the spectral MR image, improved the signal:noise ratio, and facilitated identification of MR velocity at AVO. The correlation between invasive pulmonary wedge pressure (PCWP) and noninvasive LAP (LAPni) was good, with a bias of -0.9 +/- 6.6 mmHg (+/- 2SD) and individual discrepancies of 6 mmHg or less.Conclusions: Signal averaging enhances the sharpness of the MR Doppler envelope, and facilitates the computer-assisted measurement of LAP. This novel technique may improve other quantitative methods based on the MR Doppler spectrum and make possible accurate, fully-automated LAP determination.