Purpose Three-dimensional (3D) echocardiography has been reported to be valuable for evaluating geometry for cardiac chambers. We validated the accuracy of 3D transthoracic echocardiography for quantifying aortic root geometry in comparison with cardiac multi-detector computed tomography (MDCT). Methods Twenty-three patients with normal left ventricular ejection fraction ( > 55%) as assessed by 2-dimensional transthoracic echocardiography, who underwent cardiac MDCT were enrolled (12 males, 53 짹 9 year-old). We defined the aortic root volume as the volume from aortic valvular sinuses to sinutubular junction. The measurements of aortic root volume at end diastole by both cardiac MDCT and 3DE were done offline and analyzed. Results The aortic root formed asymmetric triangle. From ventriculo-aortic junction to sinus of Valsalva, the asymmetric triangular shape was kept. From sinus of Valsalva to sinutubular junction, the shape of aortic root changed to oval shape. The average aortic root volumes measured by 3DE (ARV3DE) was 13.6 짹 4.8 ml at end diastole. The average aortic root volumes measured by 3DE was 14.1 짹 5.3 ml at end systole. The average aortic root volumes measured by MDCT (ARVCT) was 14.1 짹 5.7 ml at end diastole. The ARV3DE correlated well with ARVCT (r2=0.926, difference=0.5짹1.7ml, Figure 1). The ARV3DE showed an excellent agreement with the ARVCT (% difference = 0%, Figure 1). Conclusions Our results demonstrate the feasibility and accuracy of 3D echocardiography for clinical assessment of geometry for aortic root.