OBJECTIVES: Traditional criterion for intervention on an asymptomatic ascending aortic aneurysm has been a maximal aortic diameter of 5.5 cm or more. The 2022 American College of Cardiology/American Heart Association aortic guidelines adopted cross-sectional aortic area/height ratio, aortic size index, and aortic height index as alternate parameters for surgical intervention. The objective of this study was to evaluate the impact of using these newer indices on patient eligibility for surgical intervention in a prospective, multicenter cohort with moderate-sized ascending aortic aneurysms between 5.0 and 5.4 cm. METHODS: Patients enrolled from 2018 to 2023 in the randomization or registry arms of the multicenter trial, Treatment In Thoracic Aortic aNeurysm: Surgery versus Surveillance, were included in the study. Clinical data were captured prospectively in an online database. Imaging data were derived from a core computed laboratory. RESULTS: Among the 329 included patients, 20% were female. Mean age was 65.0 ± 11.6 years, and mean maximal aortic diameter was 50.8 ± 3.9 mm. In the one-third of all patients (n = 109) who met any 1 of the 3 criteria (ie, aortic size index ≥3.08 cm/m CONCLUSIONS: One-third of patients in Treatment In Thoracic Aortic aNeurysm: Surgery versus Surveillance would meet criteria for surgical intervention based on novel parameters versus the classic definition of diameter 5.5 cm or more. Surgical thresholds for aortic size index, aortic height index, or cross-sectional aortic area/height ratio are more likely to be met in female patients compared with male patients.