BACKGROUND: Women with thoracic aortic aneurysms within the arch or descending thoracic aorta have poorer survival than men. Sex differences in relative thoracic aortic aneurysm size may account for some of the discrepancy. The aim of this study was to explore whether basing clinical management on aneurysm size index (maximum aneurysm diameter/body surface area) rather than aneurysm size can restore equality of survival by sex. METHODS: The Effective Treatments for Thoracic Aortic Aneurysms (ETTAA; ISRCTN04044627) study was a prospective, observational cohort study. Adults referred to National Health Service hospitals in England with new/existing arch or descending thoracic aorta aneurysms greater than or equal to 4 cm in diameter were followed from March 2014 to March 2022. Baseline characteristics and survival to intervention and overall were compared for men and women. Survival models were used to assess the association between all-cause survival and sex, with and without adjustment for aneurysm diameter or aneurysm size index. RESULTS: A total of 886 thoracic aortic aneurysm patients were recruited: 321 (36.2%) women and 565 (63.8%) men. The mean(s.d.) aneurysm diameter was the same for women and men (5.7(1.1) versus 5.7(1.2) cm respectively; P = 0.751), but the mean(s.d.) aneurysm size index was greater for women than for men (3.32(0.80) versus 2.83(0.63) respectively; P < 0.001). Women had significantly worse survival without intervention: 110 (34.3%) women and 135 (23.9%) men (log rank test, P < 0.001). All-cause mortality remained greater for women after adjustment for diameter (HR 1.65 (95% c.i. 1.35 to 2.02); P < 0.001), but was attenuated after adjustment for aneurysm size index (HR 1.11 (95% c.i. 0.89 to 1.38); P = 0.359). Similar results were found for all follow-up, with or without intervention, and findings were consistent for descending thoracic aorta aneurysms alone. CONCLUSION: Guidelines for referral to specialist services should consider including aneurysm size index rather than diameter to reduce inequity due to patient sex.