Three cases are reported of acute aortic dissection with complete thrombosis of the false channel, which is a very uncommon event in De Bakey's I and II type aortic dissections. The 3 patients entered hospital because of severe thoracic pain without any ECG sign of myocardial infarction. Emergency CT showed evidence of pericardial effusion suggesting hemopericardium, enlargement of the ascending aorta and a peripheral semilunar filling defect which caused a slight deformation of the true channel. On precontrast scans, only one case showed inward displacement of peripheral intimal calcifications and high-density aortic wall. No typical signs of aortic dissection were reported, except in the first patient--where a double contrast filled lumen, separated by an intimal flap was seen. CT findings are individually discussed. It is emphasized that in all patients more than one CT sign was present at a time. The correct evaluation of these signs together with the clinical data could lead to the right diagnosis of aortic dissection in spite of the lack of a filled false channel.