A retrospective study was undertaken to evaluate the appearance of the aortic wall on computed tomography for the purpose of developing criteria for differentiating acute aortic intramural hematoma from thrombosed false lumen seen in aortic dissection. Computed tomography angiography findings of the thoracoabdominal aorta in 23 patients with suspected intramural hematoma and 25 with thrombosed false lumen were reviewed. The more common features of an intramural hematoma were hyperattenuation of the aortic wall, wall thickness less than a quarter of the aortic diameter, intrinsic wall calcification, a lesion extending around the entire aortic circumference, and ulcer-like projections that may be precursors of intramural hematoma. Wall thickness less than a quarter of the aortic diameter, lesion extending around the entire aortic circumference, and ulcer-like projections were the most useful indicators for distinguishing intramural hematoma from the thrombosed false lumen in aortic dissection.