Twenty one patients (age 4 to 25 years) with bicuspid aortic valves proved by surgery and angiography (12) or angiography alone (9) were studied by echocardiography. Seventeen patients had aortic valve disease (11 stenosis, 6 incompetence) and 4 had coarctation of the aorta with normally functioning aortic valves. A comparison group of 16 patients (9 with aortic valve disease and 7 without aortic valve disease) who had proven tricuspid aortic valves was also studied. Echocardiograms in tricuspid aortic valve patients showed the closed position of the cusps near the middle of the aortic lumen. In contrast marked eccentricity of the aortic valve cusp echoes in diastole could be demonstrated in all with bicuspid aortic valves. The Eccentricity Index (1/2 aortic lumen diameter/minimum distance of the diastolic cusp echo from the nearest aortic margin( was low (range 1.0-1.25) with tricuspid aortic valves and high (range 1.4-5.6) with bicuspid aortic valves (P <0.001). This index was not significantly affected following successful valvotomy. Bicuspid aortic valve cusps showed normal systolic motion pattern resulting in marked asymmetry of leaflet images. About half of patients with bicuspid aortic valves also showed multilayered echoes in diastole in the absence of fluoroscopic evidence of valvular calcification. Echocardiography appears to be specific in the recognition of the bicuspid aortic valve.