Aortic root diameter was measured by two-dimensional (2DE) and M-mode echocardiography in 20 adult patients (aged 29 to 69 years) undergoing aortic valve replacement in order to predict prosthetic aortic valve size. Eight patients had predominantly aortic stenosis, six had chronic aortic regurgitation, and six had acute severe aortic regurgitation secondary to infective endocarditis. 2DE measurements of aortic anulus diameter, as determined from the parasternal long-axis view, demonstrated a high correlation with actual prosthetic valve size implanted at surgery (r = 0.89, p less than 0.001, SEE 0.68 mm). 2DE exactly predicted actual prosthetic valve size in 12 of 20 patients (60%), was within 1 mm of prosthetic valve size in 6 of 20 patients (30%), and was within 2 mm of prosthetic valve size in two patients. In contrast, M-mode echocardiography failed to significantly predict aortic valve size (r = 0.14) because of its lack of two-dimensional anatomic orientation. Thus 2DE can safely and accurately predict preoperatively prosthetic aortic valve size and thereby be of great value in helping to avoid the problem of prosthesis-patient mismatch.