There has been no report in the literature of emergency vectorcardiographic study in patients with acute aortic dissection. A total of 135 consecutive patients with suspected coronary artery disease was studied. On admission to the intensive-care unit through the emergency service, each patient was given electrocardiographic and vectorcardiographic examinations, which were repeated at intervals of 24 hours. Fourteen patients (9 type I, 2 type II, and 3 type III according to DeBakey’s classification) had acute aortic dissection indicated by clinical symptoms and proven by angiography. The electrocardiograms all revealed abnormal T-wave inversion in limb or precordial leads except 3 patients. Nine (64%) of the 14 patients had abnormal length/width (L/W) ratios of the T-loop in 2 planes of the emergency vectorcardiogram. Two (22%) among these 9 patients died of ventricular tachyarrhythmia. The inscription direction of the T-loop showed abnormality in 4 (29%) of the 14 patients; 2 (50%) among these 4 patients died of ventricular tachyarrhythmia. The de novo occurrence of sustained ventricular tachyarrhythmia after emergency cardiac surgery in patients with acute aortic dissection has been documented in this report. Furthermore, patients with acute aortic dissection who have (1) an abnormal sense of inscription direction of the T-loop in at least 1 of either the horizontal or left sagittal plane, and (2) an abnormal L/W ratio in at least 2 planes in the preoperational emergency vectorcardiogram, have altered ventricular repolarization and thus are at high risk of postoperationally unexpected ventricular tachyarrhythmia. This information is not available from the electrocardiogram.