We sought to compare aortic annulus diameter measurements between trans-thoracic echocardiography (TTE) and multidetector computed tomography (MDCT), and determine the impact of discordant sizing between these two techniques on post-procedural para-valvular regurgitation (PVR) among patients undergoing trans-aortic valve implantation with a balloon expandable valve. A total of 136 consecutive patients underwent both TTE and MDCT for pre-operative assessment of aortic annulus (using mean diameter). Patients with significant renal impairement were excluded from the analysis. Prosthesis size was selected according to TTE measures. Discordant sizing referred to the situation where MDCT measures would have led to the implantation of a different size of prosthesis. We retrospectively analysed clinical and imaging data to determine variables associated with the occurrence of more than mild post-procedural PVR. Mean annulus diameters found by MDCT (CT-Mean-D) were larger than by TTE (TTE-D) (22.45±1.95mm vs. 21.33±1.78mm; p<0.001). Discordant sizing was observed in 32 patients (23.5%), and a larger valve would have been implanted in 30 patients (22%) if we had referred to MDCT sizing alone. Incidence of post-procedural PVR was higher in patients with discordant than those with consistent sizing (25% vs. 9.6%; p=0.02). In uni-variable logistic regression, the occurrence of post-procedural PVR was significantly associated with discordant sizing (OR=3.13; 95% CI: 1.12 to 8.80; p=0.03), a higher difference between CT-Mean-D and TTE-D (OR=1.80; 95% CI: 1.22 to 2.66; p=0.003 by millimeter increase) and a higher CT-Mean-D (OR=1.47; 95% CI: 1.10 to 1.99; p=0.006 by millimeter increase). Discordant sizing between TTE and MDCT is associated with more post-procedural PVR. Pre-procedural identification of these patients and reassessment with multimodality imaging may improve clinical outcomes.