Aortic valve replacement (AVR) in the patient with a small aortic root demands special consideration because the hemodynamic function of artificial valves with a small external diameter is often poor. In this study, the internal diameter of the aortic root was measured from biplane ventriculography. This measured root diameter was then used to predict the external diameter of the artificial valve. Twelve patients underwent biplane ventriculography followed by AVR with Carpentier-Edwards bioprostheses. The artificial valve diameter was predicted with a correlation coefficient of 0.93, a standard error of estimate of 0.89 mm, and an average absolute difference between preoperative measurement and valve diameter of 0.69 mm. Therefore, the aortic root diameter can be accurately measured from the ventriculogram, thus detecting the patient with a small aortic root before surgery.