Type II endoleaks are associated with the absence of aneurysm shrinkage after endovascular abdominal aortic aneurysm repair (EVAR). This study aims at determining the predictability of this complication, whose potential risk factors have been the subject of conflicting reports. Preoperative computed tomography (CT) scans of 178 patients who underwent EVAR for true infrarenal abdominal aortic aneurysms between January 20, and April 17, 2003, with a minimum follow-up of 30 days, were reviewed. The following information was retrieved: maximum aneurysm diameter, aneurysm thrombus load (maximum thickness, percentage of sac circumference wall coverage, percentage of maximum sac area occupancy); number, diameter, and nature (lumbar, inferior mesenteric, accessory renal, middle sacral) of patent aortic side-branch arteries; thrombus thickness at each aortic branch ostium, and aneurysm diameter at that level. Postoperative CT and duplex scans supplemented with angiography in selected cases were reviewed for the presence of a type II endoleak observable beyond the 30th postoperative day. Logistic regression was used to assess the association of each variable with this outcome. There were 38 (21.3%) patients with type II endoleaks after the 30th postoperative day. The median follow-up was 12 months (range 1-65 months). By univariate analysis, the following variables significantly decreased the risk of a type II endoleak: thrombus maximum thickness [odds ratio (OR) 0.77 for a 5 mm increase, p = 0.009], mean thrombus thickness at aortic side-branches ostia (OR 0.65 for a 1 mm increase, p = 0.0006), thrombus-occupied percentage of maximum aneurysm area (OR 0.72 for a 10% increase, p < 0.0001), percentage of thrombus-lined aneurysm wall (OR 0.53 for a 25% increase, p < 0.0001). The presence of a patent inferior mesenteric artery (OR 6.84, p < 0.01) and the number of patent aortic side-branches (OR 1.37 for each additional vessel, p = 0.002) significantly increased the risk of detecting a late type II endoleak. Aneurysm and aortic side-branch diameters did not have any impact. In a multiple logistic regression model (whole model p < 0.0001), the thrombus-occupied percentage of maximum aneurysm area (OR 0.74 for a 10% increase, p < 0.0005) and the number of patent aortic side-branches (OR 1.31 for each additional vessel, p = 0.009) remained independent predictors of type II endoleaks. The simple measure of the proportion of maximum aneurysm area occupied by thrombus may be a useful way to identify patients at high risk of a persistent type II endoleak. Patients with low preoperative sac thrombus load should be followed with a high degree of suspicion for this complication.