Aortic orifice area is routinely estimated by a modification of the continuity equation based on the ratio of peak subaortic and transaortic velocity. This has never been adequately validated against the classical form of the equation based on the ratio of velocity integrals. The aim of this study was to compare the 2 forms of the equation over a wide range of orifice areas in native and prosthetic valves. 106 patients were studied, 56 with native aortic stenosis (orifice area 1.0 cm^2, SD 0.4 cm^2) and 50 with normal prosthetic aortic valves (orifice area 1.6 cm^2, SD 0.5 cm^2). Velocity integral and peak velocity were averaged over the same 5 beats on both the pulsed subaortic and continuouswave transaortic recordings. The ratios of peak velocity and velocity integral correlated well (r = 0.96, s = 0.04; p < 0.0001). However, the 2 sets of values were statistically significantly different using a paired t test (T = 6.4; p < 0.00001). Agreement was reasonable at orifice areas below 1.0 cm^2 but the ratio based on peak velocity tended to underestimate at larger orifice areas. There was a weak negative correlation between the difference in the ratios and mean orifice area (r = 0.29, s = 0.04, p = 0.002). The disparity between the ratios was probably caused by changes in the shape of the continuous-wave envelope at different orifice areas. In conclusion, the modified formula is a reasonable substitute for the classical formula in patients with significant aortic stenosis but not in patients with mild stenosis or with normal prosthetic aortic valves.