Difference between arch diameter and true lumen diameter in the descending aorta was studied in patients with type B aortic dissection. The diameters of the aortic arch (Proximal ) and mid-descending aorta (Distal ) were measured on computer tomography angiography (CTA) in 20 healthy adults. Forty-two patients with type B aortic dissection who underwent endovascular repair were divided into two groups: an acute group (23 patients) and a chronic group (19 patients). The diameters of the arch (Proximal ) and the true lumen of the mid-descending aorta (Distal ) were measured on digital subtraction angiography (DSA) and CTA. The taper ratio was defined as (Proximal -Distal )/(Proximal )x100%. In the control group, the taper ratio was 13.0+/-4.7% on CTA. In the acute patients group, the taper ratio was 23.6+/-11.3% on DSA and 21.9+/-12.1% on CTA. In the chronic patients group, the taper ratio was 31.5+/-13.6% on DSA and 30.1+/-11.4% on CTA. In both acute and chronic type B aortic dissection, the aorta tapers significantly from arch to true lumen in the descending aorta. Stent-graft with tapered design may be a viable treatment option for endovascular repair of type B aortic dissection.