Thirteen cases of acute aortic dissection with non-opacified false lumen of the ascending aorta were examined by CT and other imaging modalities. On the basis of the initial CT findings, these cases were classified into two types; one was pure non-opacified dissection not associated with opacified false lumen (Type N, n = 7), the other was non-opacified dissection of the ascending aorta associated with opacified false lumen of the following aorta (Type N+O, n = 6). On examining the relation between the entry site and the false lumen in Type N+O, the dissection of the ascending aorta was considered to be retrograde. Retrograde dissection seemed to be an important factor in the development of the non-opacified dissection of the ascending aorta. During the follow-up period, re-dissection in the ascending aorta occurred in four of the 13 cases (Type N = 3, Type N+O = 1). The re-dissection occurred within the first four weeks in all of them, and the diagnosis of re-dissection was possible at its early stage. In one case, ulcerlike projection (ULP) was detected by aortography. In another case, ULP was identified by cine-MR imaging. Contrast CT also revealed enlargement and small opacification of the false lumen. In two other cases, similar CT findings were observed. Three of the four patients recovered by surgical treatment. One died the day after the diagnosis of re-dissection. Early diagnosis and earliest possible surgical intervention for re-dissection were considered necessary to save the patients with re-dissected false lumen in the ascending aorta. Close observations with several imaging modalities, mainly CT examination, should be paid in the patients with non-opacified dissection of the ascending aorta for at least four weeks after the onset of dissection.