Aortic valve replacement can result in patient-valve mismatch and attenuated left ventricular remodeling. Using CarboMedics mechanical valves, we examined if the supraannular Top Hat prosthesis (n = 7) provided a size advantage over the intraannular valve (n = 10) in patients with aortic stenosis and aortic root £23 mm. Doppler echocardiography was performed preoperatively and after three months. There no difference in aortic annulus size, but mean prosthesis size was significantly larger in Top Hat patients than in intraannular patients (25.00 mm versus 21.60 mm); the mean size improvement for Top Hat patients was 3.14 mm. After three months, all patients had excellent functional improvement and low transvalvular pressure gradients, with a slightly higher effective valve opening area in the Top Hat group. To conclude, the supraannular Top Hat valve provides an advantage of one to two sizes over the intraannular valve, and improves effective valve opening area.