Intravascular ultrasound (IVUS) has been successfully used to guide the implantation of stents in the thoracic aorta. However, its accuracy in measuring the diameter of the aortic lumen has not been clearly established. Thirteen patients with thoracic aortic disease underwent IVUS, and lumen diameter measurements were compared with those obtained by CT or magnetic resonance imaging. A total of 31 comparable measurements were obtained. The correlation was good (r=0.98; P< .001), with IVUS tending to give a larger minimum diameter than CT (systematic error, 0.59+/-1.8 mm; P=.077). Given that the aorta is often not circular, the diameter obtained by IVUS was also compared to the mean diameter obtained by CT, and it was found that these two measurements were more closely related (P=.425), except in aortic segments with significant eccentricity (i.e., >10%). In conclusion, IVUS was a reliable tool for measuring the diameter of the aorta, particularly in concentric segments where stents are normally placed. Consequently, IVUS could supplement conventional imaging techniques.