This study evaluated a strategy that makes optimal use of Doppler echocardiography for estimating the severity of valvular aortic stenosis (AS). Fifty-eight patients with no more than moderate aortic insufficiency who underwent cardiac catheterization were evaluated with two-dimensional echocardiography and Doppler velocimetry to determine the peak velocity across the stenotic valve and aortic valve area (AVA) by means of the continuity equation. All 33 peak Doppler velocities of greater than or equal to 4 m/sec had critical AS (AVA less than or equal to 0.8 cm2 at catheterization). Conversely, six of seven patients with Doppler velocities of less than or equal to 3 m/sec had noncritical AS. The patient with a falsely low peak velocity had severely depressed left ventricular function. Doppler velocity alone was inadequate in determining severity of AS for patients with velocities between 3 and 4 m/sec. The continuity equation proved accurate in estimating AVA in the 46 patients for whom catheterization and ultrasound data were sufficient to compare calculated AVA (r = 0.81), and was also accurate for those patients with peak Doppler velocities between 3 and 4 m/sec (r = 0.90). These results suggest that Doppler velocimetry alone is adequate in determining critical vs noncritical AS in many patients, while the continuity equation should be applied for patients with peak velocities between 3 and 4 m/sec as well as in patients with severely depressed cardiac function.