The preoperative 2-dimensional (2-D) echocardiograms of all patients less than 50 years of age in whom the aortic valve had been directly inspected by the surgeon or the pathologist or both were reviewed. From June 1977 to June 1981, 283 patients aged less than or equal to 50 years had aortic valve surgery at the Mayo Clinic: 115 (aged 1 to 50 years [mean 32]) had 2-D examinations preoperatively. The echocardiograms were reviewed blindly, and the aortic valve structure was categorized as bicuspid, tricuspid, or indeterminate. On the basis of combined surgical and pathologic inspection, 50 aortic valves were congenitally bicuspid, 60 were tricuspid, 4 were unicommissural, and 1 was quadricuspid. By 2-D echocardiography, the number of cusps was indeterminate in 29 patients (25%). When these patients were excluded, the sensitivity, specificity, and diagnostic accuracy of 2-D echocardiography for bicuspid aortic valve were 78,96, and 93%, respectively. Thus, with adequate 2-D images, echocardiography is a sensitive and highly specific technique for the diagnosis of bicuspid aortic valve.