Contrast-enhanced CT images in patients with acute or chronic aortic dissection were evaluated for the purpose of determining the clinical usefulness of "aortic cobwebs", which were originally reported by Williams et al. as an anatomical marker of the false lumen. Aortic cobwebs were detected in 13 of 44 cases (30%), demonstrating as low density structures in the false lumen continuing from the intimal flap. The aortic cobwebs were mainly linear in shape, and the size ranged from 5 x 1 to 15 x 3 mm (mean: 9 x 2 mm). Aortic cobwebs were more frequently demonstrated in chronic than in acute cases, and were more often detected in the abdominal aorta. Follow-up CT examinations of the cobwebs revealed no interval change in six cases, disappearance in three cases and thickening in one case. There was no significant correlation between the sequential changes in the cobwebs and dilatation or thrombus formation in the false lumen. Aortic cobwebs seem to represent partially residual bands of the aortic media incompletely sheared from the aortic wall during the initial processes of aortic dissection. These structures are considered to be a reliable anatomic marker and to be useful for CT identification of the false lumen in aortic dissection.