This study aims to outline the ‘true’ natural history of ascending thoracic aortic aneurysm (ATAA) based on a cohort of patients not undergoing surgical intervention. The outcomes, risk factors, and growth rates of 964 unoperated ATAA patients were investigated, over a median follow-up of 7.9 (maximum of 34) years. The primary endpoint was adverse aortic events (AAE), including dissection, rupture, and aortic death. At aortic sizes of 3.5–3.9, 4.0–4.4, 4.5–4.9, 5.0–5.4, 5.5–5.9, and ≥6.0 cm, the average yearly risk of AAE was 0.2%, 0.2%, 0.3%, 1.4%, 2.0%, and 3.5%, respectively (P < 0.001), and the 10-year survival free from AAE was 97.8%, 98.2%, 97.3%, 84.6%, 80.4%, and 70.9%, respectively (P < 0.001). The risk of AAE was relatively flat until 5 cm of aortic size, at which it began to increase rapidly (P for non-linearity <0.001). The mean annual growth rate was estimated to be 0.10 ± 0.01 cm/year. Ascending thoracic aortic aneurysms grew in a very slow manner, and aortic growth over 0.2 cm/year was rarely seen. Multivariable Cox regression identified aortic size [hazard ratio (HR): 1.78, 95% confidence interval (CI): 1.50–2.11, P < 0.001] and age (HR: 1.02, 95% CI: 1.00–1.05, P = 0.015) as significant independent risk factors for AAE. Interestingly, hyperlipidemia (HR: 0.46, 95% CI: 0.23–0.91, P = 0.025) was found to be a significant protective factor for AAE in univariable Cox regression. An aortic size of 5 cm, rather than 5.5 cm, may be a more appropriate intervention criterion for prophylactic ATAA repair. Aortic growth may not be an applicable indicator for intervention. To assess the natural history of ascending aortic aneurysm, we examined the three-decade clinical outcomes of 964 non-operated patients with ascending aortic aneurysm. Note, in the 2D plot, the sharp rise in risk of adverse aortic events is at an aortic size of 5 cm. The 3D plot of risk vs. both size and age also discloses a substantial contribution of advancing age to increased risk. ATAA, ascending thoracic aortic aneurysm; AAE, adverse aortic events.