Objectives To investigate changes in aortic diameter in hemodynamically unstable trauma patients and the implications for sizing of thoracic endovascular aortic repair (TEVAR) in patients with traumatic thoracic aortic injury (TTAI). Methods We retrospectively evaluated all trauma patients that were admitted with hemodynamic instability (mean arterial pressure <95 mm Hg and a pulse ≥100 beats/min) and underwent computed tomography (CT) of the thorax and abdomen both at admission and at another moment (control CT scan), at the Yale New Haven Hospital between 2002 and 2009. The CT examinations were reviewed in a blinded fashion and the aortic diameter was measured at six different levels by a cardiovascular radiologist. Differences in aortic diameter between the initial CTs obtained in the trauma bay and the control CTs were compared using the paired Student t test. Results Forty-three patients were identified, including 32 males. Mean age was 37 ± 16 years, mean injury severity score was 26 ± 15, the mean pulse and blood pressure were 122 beats/min and 103/63 mm Hg, respectively. Overall, the mean aortic diameter was significantly larger at the control CT examinations compared with the initial CT examinations while hemodynamically unstable, at all evaluated levels. Among patients with a pulse ≥130/min, the mean increase in aortic diameter was most consistent at the level of the mid descending thoracic aorta (DTA, +12.6%, P = .003) and at the level of the infrarenal aorta (+12.6%, P = .004). Conclusions The aortic diameter decreases dramatically in trauma patients with hemodynamic instability. This decrease in aortic diameter could theoretically lead to inaccurate aortic measurements and undersizing of the endograft in hemodynamically unstable TTAI patients requiring TEVAR. Further research is needed to better predict the actual aortic diameters in individual hemodynamically unstable patients requiring endovascular aortic repair.