Patients undergoing endovascular aneurysm repair (EVAR) for abdominal aortic aneurysms can exhibit variations in sac behavior ranging from complete regression to expansion. We evaluated the impact of sac behavior at 1-year follow-up on late survival. We used the Vascular Study Group of New England (VSGNE) registry from 2003 to 2011 to identify EVAR patients with 1-year computed tomography follow-up. Aneurysm sac enlargement ≥5 mm (sac expansion) and decrease ≥5 mm (sac regression) were defined per Society for Vascular Surgery guidelines. Predictors of change in sac diameter and impact of sac behavior on long-term mortality were assessed by multivariable methods. Of 2437 patients who underwent EVAR, 1802 (74%) had complete 1-year follow-up data and were included in the study. At 1 year, 162 (9%) experienced sac expansion, 709 (39%) had a stable sac, and 931 (52%) experienced sac regression. Sac expansion was associated with preoperative renal insufficiency (odds ratio [OR], 3.4; 95% confidence interval [CI], 1.5-8.0; P < .01), urgent repair (OR, 2.7; 95% CI, 1.4-5.1; P < .01), hypogastric coverage (OR, 1.7; 95% CI, 1.1-2.7; P = .02), and type I/III (OR, 16.8; 95% CI, 7.3-39.0; P < .001) or type II (OR, 2.9; 95% CI, 2.0-4.3; P < .001) endoleak at follow-up, and sac expansion was inversely associated with smoking (OR, 0.6; 95% CI, 0.4-0.96; P = .03) and baseline aneurysm diameter (OR, 0.7; 95% CI, 0.6-0.9; P < .001). Sac regression (vs expansion or stable sac) was associated with female gender (OR, 1.8; 95% CI, 1.4-2.4; P < .001) and larger baseline aneurysm diameter (OR, 1.4; 95% CI, 1.2-1.5; P < .001) and inversely associated with type I/III (OR, 0.2; 95% CI, 0.1-0.5; P < .01) or type II endoleak at follow-up (OR, 0.2; 95% CI, 0.2-0.3; P < .001). After risk-adjusted Cox regression, sac expansion was independently associated with late mortality (hazard ratio, 1.5; 95% CI, 1.1-2.0; P = .01), even with adjustment for reinterventions and endoleak during follow-up. Sac regression was associated with lower late mortality (hazard ratio, 0.6; 95% CI, 0.5-0.7; P < .001). Long-term survival was lower (log-rank, P < .001) in patients with sac expansion (98% 1-year and 68% 5-year survival) compared with all others (99% 1-year and 83% 5-year survival). These data suggest that an abdominal aortic aneurysm sac diameter increase of at least 5 mm at 1 year, although infrequent, is independently associated with late mortality regardless of the presence or absence of endoleak and warrants close observation and perhaps early intervention. Copyright © 2017 Society for Vascular Surgery. Published by Elsevier Inc. All rights reserved.