This prospective, blinded study evaluated the accuracy of transesophageal (TEE) and transthoracic echocardiography in predicting the size of the surgical aortic valve anulus in 20 consecutive patients undergoing aortic valve replacement. TEE was highly accurate in predicting surgical aortic valve anulus size (r = 0.98; p < 0.00001). The standard error of the TEE estimate of the size of the obturator anulus was only 0.04 mm, and in all cases the TEE estimate was within 1 mm of the direct surgical measurement. Both biplane and multiplane TEE probes were equally accurate and this accuracy was maintained over a wide range of sizes of surgical aortic anulus. Transthoracic echocardiography appeared less accurate in predicting anulus size (r = 0.64; p < 0.08). Therefore TEE can be used routinely for selection and preparation of aortic allografts before sternotomy, potentially reducing cardiopulmonary bypass pump time by 10 to 30 minutes and ensuring optimal size matching of the donor-recipient anulus.