Background: Subsequent intensive care unit (ICU) admissions postoperatively are not rare for patients with abdominal or thoracic aortic aneurysm (AAA or TAA), but a large-scale investigation on these patients is absent. The study aimed to investigate the characteristics and prognosis of AAA or TAA patients admitted to ICU postoperatively.Methods: Patients admitted to ICU postoperatively with a primary diagnosis of AAA or TAA were screened in the eICU Collaborative Research Database, which contained data from multiple ICUs throughout the continental United States in 2014 and 2015. Baseline characteristics and comorbidities and were investigated and factors associated with ICU mortality were explored using univariable logistic regression. Receiver operating characteristic (ROC) curve analysis was performed to evaluate the prognosis predictive performance of the widely used severity scoring system APACHE IVa.Results: 974 patients including 677 AAA and 297 TAA patients admitted to ICU postoperatively were included finally. Compared with TAA, AAA patients had a significantly higher median age (72 versus 64 years). 10.19% AAA and 2.36% TAA patients suffered from rupture of aortic aneurysm, and 89.07% AAA and 84.51% TAA patients underwent elective surgery. Hypertension requiring treatment was the most common comorbidity (57.31% for AAA and 61.95% for TAA). TAA patients had significantly higher ICU mortality (9.43% versus 2.36%) than AAA. Several factors were found to be significantly associated with ICU mortality, including urgent surgery, rupture of aortic aneurysm, TAA, and a higher APACHE IVa score on ICU admission. APACHE IVa showed a good predictive performance for ICU mortality with an area under the ROC curve of 0.9176 (95% CI 0.8789-0.9390).Conclusions: Prognosis of aortic aneurysm patients admitted to ICU postoperatively is yet to improve, and factors associated with prognosis are mainly related to the condition itself. APACHE IVa can be used for prognosis prediction.