ObjectivesGuidelines for surgical correction of patients with ascending thoracic aortic aneurysm (ATAA) with a bicuspid aortic valve (BAV) have oscillated over the years. In this study, we outline the natural history of the ascending aorta in patients with BAV and trileaflet aortic valve (TAV) ATAA followed over time, to ascertain if their behavior differs and to determine if a different threshold for intervention is required. MethodsAortic diameters and long-term complications (ie, adverse aortic events) of 2428 patients (554 BAV and 1874 TAV) with ATAA before operative repair were reviewed. Growth rates, yearly complication rates, event-free survival, and risk of complications as a function of aortic size were calculated. Long-term follow-up and precise cause of death granularity was achieved via a comprehensive 6-pronged approach. ResultsAortic growth rate in patients with BAV vs TAV ATAA was 0.20 and 0.17 cm/year, respectively (P = .009), with the rate increasing with increasing aortic size. Yearly adverse aortic events rates increased with ATAA size and were lower for patients with BAV. The relative risk of adverse aortic events exhibited an exponential increase with aortic diameter. Patients with BAV had a lower all-cause and ascending aorta-specific adverse aortic events hazard. Age-adjusted 10-year event-free survival was significantly better for patients with BAV, and BAV emerged as a protective factor against type A dissection, rupture, and ascending aortic death. ConclusionsThe threshold for surgical repair of ascending aneurysm with BAV should not differ from that of TAV. Prophylactic surgery should be considered at 5.0 cm for patients with TAV (and BAV) at expert centers.