The accuracy of combined M-mode and two-dimensional echocardiography in the diagnosis of aortic dissection was evaluated in 673 patients with a clinical suspicion of aortic dissection, over a six-year period. In 128 cases, the diagnosis of aortic dissection was confirmed by angiographic, tomographic (CT scan), or autopsy findings, or during surgery. Two echocardiographic features were found to support a diagnosis of aortic dissection: a dilation of at least one segment of the aorta (sensitivity 95%, specificity 51%) and a typical abnormal linear intraluminal echo corresponding to the intimal flap (sensitivity 67%, specificity 100%). This pathognomonic intimal flap was observed in 86 cases, of which three types could be distinguished: (1) a long oscillating flap (n = 15), (2) a long but minimally mobile linear echo which was duplicated and parallel to one or two aortic walls (n = 64), (3) a short, double linear image with a rapid systolic motion and high frequency oscillations. These features were found to have a high sensitivity in type I aortic dissection (88%), although in types II and III the sensitivity was much lower. In some cases, a fourth type of abnormal image could be detected: a small intraluminal echo moving in parallel to the aortic wall. This feature should be interpreted with caution since its predictive value for a positive examination was low (48%). Out of 23 cases in which the diagnosis of aortic dissection was suspected on the basis of this doubtful abnormal echo, it was confirmed in only 11 patients. The results in these 128 cases of aortic dissection indicate that two-dimensional echocardiography, which is easily performed at the patient's bedside, could take priority in investigations of this condition. It is extremely sensitive in the diagnosis of ascending aortic dissection, but much less so in the diagnosis of descending aortic dissection.