This study describes a method for estimation of the ratio of pulmonary to systemic pressures by pulsed-wave Doppler echocardiography. Sixty-eight patients ages 1 day to 68 years who underwent cardiac catheterization had Doppler studies of the right and left ventricular outflows. Preejection period (PEP), ejection time (ET) and mean acceleration to peak velocity (ACCm) were measured on each waveform. The expression: F = (PEP x ACCm)/ET was calculated for right and left ventricular outflows as an index of the effects that the interaction between ventricular contraction and afterload has on the shape of the Doppler waveforms generated in each outflow. The quotient of (F for the right outflow)/(F for the left outflow), or waveform contour ratio, was used to express the degree of pressure-dependent variability between each subject's right and left ventricular outflow tracings. The waveform contour ratio was strikingly similar to the ratio of systolic pulmonary to systemic pressures and also closely correlated to the ratio of mean pressures. The product of waveform contour ratio and arm systolic pressure gave a consistently accurate estimate of systolic pulmonary pressures. It is concluded that the present method can be used successfully for the noninvasive assessment of pulmonary arterial pressures.