We evaluated the residual false lumen of type I and IIIb dissecting aneurysm by CT, MRI and angiography postoperatively. The 19 patients with type I dissecting aneurysm were included eleven men and eight women, the average age was 55.8 +/- 10.2 years old. The 20 patients with type IIIb dissecting aneurysm were included sixteen men and four women, the average age was 56.2 +/- 8.5 years old. The rate of distal patent false lumen was 52.6% of type I and 35% of type IIIb dissecting aneurysm after graft replacement surgery. In type I dissecting aneurysm, the rate of distal patent false lumen was 40% of acute stage vs 66.7% of chronic stage, 66.7% of ascending and partial arch replacement vs 46.2% of ascending and total arch replacement, and 90% of graft inclusion technique vs 11.1% of graft exclusion technique. The distal patent false lumen was the lowest (12.5%) with type I dissecting aneurysm of ascending and total arch replacement using graft exclusion technique. In type IIIb dissecting aneurysm, the rate of patent false lumen was 66.7% of acute stage vs 29.4% of chronic stage, 30% of graft exclusion technique vs 40% of graft inclusion technique. The size of false lumen preoperatively were larger (11.1 +/- 4.5 cm2) in patients with distal patent false lumen than that (6.7 +/- 3.2 cm2) of in patients with distal occlusive false lumen.(ABSTRACT TRUNCATED AT 250 WORDS)