Six patients with aortic root dissection confirmed by angiography, surgery, or autopsy were studied by echocardiography. All showed marked parallel widening of the anterior (16 to 21 mm) and/or posterior (10 to 13 mm) aortic walls together with enlargement of the aortic root image. (Normal mean aortic wall thickness 5.7 mm, SD 1.2; aortic valve disease patients 6.7 mm, SD 1.5). In five patients slender aortic valve cusps were recorded moving to the periphery of the inner lumen in systole and not extending to the outer lumen. This finding is useful in excluding calcification of the aortic valve which may produce confusing multiple echoes within the aortic root. Two patients showed 8 to 20 mm variations in the width of the aortic image with slight change in the direction of the transducer indicating that the dissecting hematoma was not uniform in thickness. Other associated findings on the echogram seen in three patients included the demonstration of pericardial fluid collection and mitral diastolic flutter suggestive of aortic regurgitation. Demonstration of enlargement of the aortic root with marked parallel widening of anterior and/or posterior walls appears to be specific for aortic root involvement in dissecting aneurysm of the aorta.