OBJECTIVE The expansion of aneurysms after endovascular repair is a consequence of persistent sac pressure, usually resulting from an endoleak. Several authors have suggested that sac expansion can occur even in the absence of endoleak, referring to this phenomenon as endotension. This study undertakes a review of the largest US endograft trial data to better define the significance of aneurysm expansion in the absence of endoleak. METHODS The core laboratory imaging database from the Ancure (Guidant) endovascular graft Phase I and Phase II trials approved by the Food and Drug Administration was reviewed with attention to aneurysm size and endoleak. Aneurysm size was measured with standardized two-dimensional computed tomography (CT) scan at the area of largest initial aneurysm diameter. Endoleak was detected with CT scans, color duplex ultrasound scans, and angiography in selected cases. Patients were evaluated at baseline, 3 months, 6 months, 12 months, and every 12 months thereafter. An endograft was classified as leaking if any endoleak was detected with any modality at any time point. RESULTS A total of 658 patients were entered into these protocols and the data submitted to the core laboratory. A control group of 120 conventional aortic patients and a group of 62 without baseline CT data were excluded from further analysis. Of the remaining 476 patients, 144 (60 tube, 60 bifurcated, and 24 mono-iliac) were free of endoleak at all intervals and had baseline CT measurements to allow comparison. Overall, the average size decrease in this nonleaking group was 9.9 +/- 9.4 mm (range, -50.6-11.1 mm) at a mean follow-up of 23.3 months. Evaluation for overall aneurysm expansion revealed 17 patients who had an increase of 2.3 +/- 2.9 mm (range, 0.3-11.1 mm) at a mean follow-up of 14.1 months. Only two patients without evidence of endoleak exhibited growth of more than 5 mm at maximum follow-up (7.6 mm at 12 months and 11.1 mm at 36 months). Additional analysis of sealed endoleaks and late endoleaks failed to demonstrate any group with expansion in the absence of detectable endoleak. CONCLUSIONS Endotension appears to be rare in this large series of prospectively evaluated endografts. From this review, endotension seems more likely to represent missed endoleak than true aneurysm expansion in the absence of perigraft flow.