PURPOSE: The purpose of this study was to investigate the frequency, site and characteristics of motion artifact of ascending aorta mimicking dissection. MATERIALS AND METHODS: The authors evaluated postcontrast CT scans of 60 cases in 60 patients without symptoms of aortic dissection or aortic disease. A Toshiba 900S scanner was used, with 1 cm slice thickness and 1 sec scan time. Streak artifacts, and those relating to extra-aortic vascular structure or pericardial effusion were excluded from this study. RESULTS: Crescent-shaped motion artifacts were seen in 54 cases (90%), and occurred from 1 cm to 4 cm above the level of the aortic valve; between men and women, there was no significant difference in frequency. In each case, the artifact was seen at 1 to 5 (mean 2.9) levels. Its pattern was symmetric in 31 of 60 cases (51%), and at 15 other sites, symmetric artifacts were seen between the SVC and ascending aorta. At the margin of the aortic circumference, the direction of the artifact was left anterior-right posterior in 23.9% of cases, and anterior median-posterior median in 20.8%. CONCLUSION: On CT, motion artifact of ascending aorta occurs frequently. Findings relating to location, symmetric pattern and characteristic direction of artifact may be helpful in the differential diagnosis of aortic dissection and aortic motion artifact.