In 17 patients (mean age 60 years) a thoracic aortic dissection could be revealed by intraarterial digital subtraction angiography with exact delination of the proximal and distal extent. Futhermore a visualisation of the true and false lumen and hereby especially the blood flow and its direction in the false lumen could be made. The disclosure of the intimal flap could be seen in patients with flow in both channels, separating the true and the false lumen. In 4 patients the entry could be seen sharply as a hole, but not corresponding to the proximal extent. In 11 patients the entry was seen over a longer area and in 2 patients neither entry nor reentry could be seen. The most distal part of the intimal flap might correspond to the reentry. One anonymous artery, five renal arteries and one coeliac trunc were involved by the dissection. The exact anatomical and pathophysiological nature of the dissection made it possible to decide whether or not the patients should be operated upon and if so, what kind of operation should be choosen.