Objective Short neck length, and angulated infrarenal aortic necks, are historically associated with sac expansion following endovascular abdominal aortic aneurysm repair. Aorfix (Lombard Medical, Inc, Irvine, Calif), a conformable stent graft US Food and Drug Administration approved for highly angulated necks, was investigated in the US Pythagoras Trial. We investigated the incidence of, and factors related to, sac regression and expansion following implantation of the Aorfix device. Methods Pythagoras trial data was assessed with mutivariable modeling for factors associated with sac regression or expansion (defined as change in maximum aneurysm diameter of ≥5 mm on annual follow-up computed tomography compared with last preoperative scan). All anatomic measurements were performed per protocol by a core laboratory (M2S; West Lebanon, NH). Results A total of 218 patients were enrolled; neck angle was ≥60° in 151 (69.3%), <60° in 67 (30.7%). Follow-up, including computed tomography imaging, was available at 1 year (n = 165), 2 years (n = 142), 3 years (n = 106), and 4 years (n = 70). Sac size decreased or was unchanged in 98.8% (1 year), 95.1% (2 years), 94.3% (3 years), and 92.9% (4 years). Mean neck length for the entire cohort was 22.3 mm; mean proximal neck angle was 71.4°. Maximum aneurysm diameter was 57.5 mm. Sac regression or expansion was not statistically associated with neck length, neck diameter, maximal aneurysm diameter, or patient sex (or 19 other demographic and medical history variables). Neck angle was not statistically associated with sac regression or expansion. Conclusions In this study cohort, there is a low incidence of sac expansion with the Aorfix device. We were unable to demonstrate an association between neck length/angulation and sac expansion.