OBJECTIVE--To determine late patency of the aortic false lumen and propensity for aneurysm formation after repair of type A dissection. DESIGN--Retrospective follow up study. SETTING--Regional cardiac surgical unit. PATIENTS--28 patients after repair of type A dissection. METHODS--Magnetic resonance imaging (MRI) was performed between 6 weeks and 12 months after operation. RESULTS--A patent distal false lumen with demonstrable blood flow was found in 22 patients (78%). Only six patients had complete obliteration of the false lumen by thrombus. The conduct of operation did not influence this. Nine patients (32%) showed aneurysmal dilatation of the false lumen and three had a repeat operation. CONCLUSIONS--So-called "successful repair" of aortic dissection does not obliterate the distal false lumen. MRI is a safe and effective radiological procedure for determining patency and dilatation in the false lumen. Patients with type A dissection with blood flow in the false lumen should be studied every 6 months to look for aneurysmal dilatation.