Purpose: To evaluate risk factors for early (<30 days) type I endoleak following thoracic endovascular aortic repair (TEVAR). Methods: A retrospective study was conducted of 439 consecutive patients (mean age 74.0±10.0 years; 333 men) who underwent TEVAR at a single center between June 2006 and June 2013. Pathologies included 237 aortic arch aneurysms and 202 descending thoracic aortic aneurysms (dTAA). Maximum TAA diameter was 63.6±13.7 mm. Among the distal aortic arch aneurysms, 124 required coverage of the left subclavian artery (LSA), while the remaining 113 arch aneurysms had debranching (n=40), the chimney technique (n=52), and a branched stent-graft (n=13). Eight patients with dilatation of the ascending aorta underwent arch replacement with elephant trunk prior to TEVAR. Predictive factors for type I endoleak were explored in univariate analysis and examined for each outcome using logistic regression models; results are given as the odds ratio (OR) and 95% confidence interval (CI). Results: Among 439 TEVAR cases, 37 (8.4%) had type I endoleaks on imaging at 1 month; 31 were in the 237 arch cases (13.1%). Endoleak investigation by site indicated a low incidence (3.0%) for dTAAs and markedly low (1.4%) in zone 4. Significantly more endoleaks were observed in zones 0–2 than in zone 4 (p<0.001). On univariate analysis, significant associations were found between endoleak and LSA coverage (OR 5.8, 95% CI 2.4 to 14.4, p<0.001), operative time ≥240 minutes (OR 3.7, 95% CI 1.5 to 6.2, p=0.002), and ≥270 mL of contrast (OR 2.8, 95% CI 1.4 to 5.8, p=0.004). Among the aortic branch reconstruction procedures, the chimney technique was the only maneuver associated with a significant risk of endoleak (OR 5.3, 95% CI 2.3 to 11.2, p<0.001). Arch state was not correlated with endoleaks, but ≥38-mm proximal neck diameter (OR 3.6, 95% CI 1.2 to 10.8, p=0.023), stent-graft diameter ≥40 mm (OR 9.9, 95% CI 1.4 to 30.5, p=0.015), and excessively oversized (≥14%) stent-grafts (OR 3.5, 95% CI 1.2 to 10.3, p=0.020) were; the proximal neck length was not correlated with endoleaks if a proximal neck length >10 mm can be secured. Conclusion: Risks for early type I endoleaks after TEVAR for aneurysm were landing zone 0–2, LSA coverage, large proximal neck and stent-graft diameters, excessive oversizing, and the use of the chimney technique.