A 33-year-old man with known annuloaortic ectasia was admitted to the hospital because of chest pain. A dissecting aortic aneurysm was strongly suspected, but angiography was not performed because of his history of allergic reaction to iodine contrast material. Two-dimensional echocardiography demonstrated a markedly dilated basal aorta. The aortic wall immediately above the right sinus of Valsalva appeared to divide the aortic lumen into anterior and posterior channels. A defect in the intimal flap with a diameter of two cm was also detected six cm above the aortic ring. Color Doppler flow mapping showed blood flow passing through a probable entrance tear from the true lumen into the false lumen. Reentry could not be detected by Doppler imaging. All major aortic tributaries were presumed patent and supplied via the true lumen. The patient underwent successful emergency Bentall surgical procedure and recovered. It is emphasized that in some cases of DeBakey type I or II aortic dissection, color Doppler flow imaging is helpful for determining indications for emergency surgery.