With M-mode echocardiography a bicuspid aortic valve can be diagnosed by its eccentric diastolic echoes, particularly when the eccentricity index exceeds 1, 3. However, many false positives and false negatives have been reported. Two-dimensional echocardiography, which identifies the commissures and the closure lines, can make the diagnosis more accurate. With this technique we studied 15 patients with a bicuspid aortic valve and 20 normal subjects. We found 3 different patterns according to the position of the closure line, which can be "horizontal", "vertical" and "oblique". Whenever the closure line is "horizontal", that is perpendicular to the direction of the ultrasonic beam, the M-mode echoes from the valve in diastole appear to be centrally located within the aortic root (false negatives). On the contrary, if the closure line is "vertical", that is parallel to the echo beam, "the echoes" are eccentric (diagnostic images). When the closure line is obliquely oriented, the ultrasonic images can be either central or eccentric. Two-dimensional echocardiography identified 5 "horizontal", 3 "vertical" and 7 "oblique" closure lines. It thus helped clarify the M-mode errors and proved to be more reliable for the diagnosis of bicuspid aortic valve.