In 35 patients with aortic stenosis the Doppler-derived values of the aortic valve area (continuity equation) were compared with those determined at cardiac catheterization (Gorlin's formula). The comparison of three modifications of the continuity equation showed that the procedure generally proposed (calculating the area of the left ventricular outflow tract from its diameter) significantly underestimated the valve area (modification 1). Modification 2, which used direct planimetry of the left ventricular outflow tract, yielded results quite consistent with invasive measurements. The employment of peak velocities instead of velocity-time integrals (modification 3) did not significantly alter the results. However, the scatter was considerable in all three modifications. When critical aortic stenosis was defined with a valve area less than or equal to 0.70 cm2, modifications 1, 2, and 3 accurately predicted the severity of stenosis in 80, 86, and 80%, respectively.