Purpose: To investigate the initial proximal position and seal of the Nellix EndoVascular Aneurysm Sealing (EVAS) system in the aortic neck using a novel methodology. Methods: Forty-six consecutive patients who underwent elective EVAS for an abdominal aortic aneurysm were retrospectively selected and dichotomized into an early (n=23) and a late (n=23) group. The aortic neck morphology and aortic neck surface (ANS) were determined on preoperative computed tomography (CT) scans; the endograft position and nonapposition surface (NAS) were determined on the 1-month CT scans. The position of the proximal endobag boundary was measured by 2 experienced observers to analyze the interobserver variability for the EVAS NAS measurements. The shortest distance from the lowest renal artery to the endobag (shortest fabric distance) and the shortest distance from the endobag to the end of the infrarenal neck (shortest sealing distance) were determined. The intraclass correlation coefficients (ICCs) are presented with the 95% confidence interval (CI). Continuous data are presented as the median and interquartile range (IQR: Q3 – Q1). Results: There were no differences between the early and late EVAS groups regarding aortic neck morphology except for the neck calcification circumference [41° (IQR 33°) vs 87° (IQR 60°), respectively; p=0.043]. Perfect agreement was observed for the NAS (ICC 0.897, 95% CI 0.780 to 0.956). The NAS as a percentage of the preoperative ANS was 47% (IQR 43) vs 49% (IQR 49) for the early vs late groups, respectively (p=0.214). The shortest fabric distances were 5 mm (IQR 5) and 4 mm (IQR 7) for the early and late groups, respectively (p=0.604); the shortest sealing distances were 9 mm (IQR 13) and 16 mm (IQR 17), respectively (p=0.066). Conclusion: Accurate positioning of the Nellix EVAS system in the aortic neck may be challenging. Despite considerable experience with the system, still around half of the potential seal in the aortic neck was missed in the current series, without improvement over time. This should be considered during preoperative planning and may be a cause of a higher than expected complication rate. Detailed post-EVAS nonapposition surface can be determined with the described novel methodology that takes into account the sometimes irregularly shaped top of the sealing endobags.