Thirty-four patients with acute aortic dissection with thrombosed false lumen were reviewed. In 4 cases (12%), the false lumens changed to the patent type. Changes in the false lumens were revealed by contrast-enhanced CT performed from day 2 to day 19 (mean, day 12.5) after the onset. The entry site was located at the descending aorta in 3 cases, and was unknown in 1 case. In 2 cases, the entry site corresponded with ulcer-like projections that had been pointed out at the thrombosed false lumen. One patient presented with moderate back pain when the false lumen changed, while the others were symptom free. During the follow-up period, 2 patients died of rupture of the false lumen. One patient was treated with a surgical operation. The patent false lumen became re-thrombosed in 1 patient. Although the majority of cases of thrombosed aortic dissection are considered to have a good prognosis, some cases change to the patent type without clinical symptoms. Radiologists should be aware of the unstable nature of the thrombosed type aortic dissection, and follow-up examinations should be performed at least once a week for three weeks after the onset of dissection.