In patients with aortic valve stenosis, the quantification of stenosis is usually performed using fluid-filled catheters and a computerized calculation program. The aim of this study was to determine the accuracy of this technique in comparison to the manual planimetry of the area between the curves of a simultaneous registration, using a multitip micromanometer catheter. The study was performed in 19 patients, in whom left and right heart catheterization was warranted. Systolic left ventricular and aortic peak pressures were significantly overestimated using a fluid-filled catheter (206 ؎ 35 vs. 199 ؎ 37 mm Hg, P ؍ 0.0003, and 148 ؎ 18 vs. 143 ؎ 21 mm Hg, P ؍ 0.0052). However, peak-to-peak pressure gradients were identical comparing both techniques (58 ؎ 31 vs. 56 ؎ 32 mm Hg, r ؍ 0.983). The mean pressure gradients and aortic valve areas based on simultaneous measurements of left ventricular and aortic pressures by micromanometer catheters were identical to the values determined by a computer-based program using fluid-filled catheters (54 ؎ 21 vs. 52 ؎ 21 mm Hg, r ؍ 0.923, P F 0.05, and 0.75 ؎ 0.25 vs. 0.77 ؎ 0.25 cm 2 , r ؍ 0.935). Thus, the conventional use of fluid-filled catheters and of a computerized calculation of aortic valve area is valid for quantification of aortic stenosis in patients with sinus rhythm and without significant aortic regurgitation.