Purpose To evaluate significant factors related to delayed aortic false lumen (FL) enlargement in patients who have undergone thoracic stent-graft placement for type B aortic dissection. Materials and Methods The study included 62 patients (45 male, 17 female) aged 26–80 years (mean age, 58.1 y) who underwent thoracic endovascular aortic repair for type B aortic dissection at a single institution between January 2005 and May 2015. Mean age of aortic dissections was 5.3 months (range, 0.1–73.3 mo). Maximum aortic diameter at presentation was 41.7 mm ± 8.3. The follow-up period ranged from 3 to 104 months (mean, 27.1 mo). Computed tomographic (CT) angiography studies were reviewed to identify FL diameter enlargements > 5 mm at different levels along and distal to the stent graft. Imaging findings and clinical variables were investigated to determine their correlation with FL enlargement. Results No significant difference was found between the ages of aortic dissections in patients with and without FL enlargement (P = .26). On follow-up CT angiography, 16 patients had 2 or more communication channels between the FL and the systemic circulation, 7 of whom showed FL enlargement > 5 mm (P = .007). Twenty-seven patients showed complete FL thrombosis, none of whom had FL enlargement (P < .001). Conclusions Two or more communication channels between the FL and the systemic circulation represent a risk factor for FL enlargement regardless of the age of the dissection. Patients with thrombosis of the FL are less likely to experience FL enlargement.