Ascending aortic length has recently been recognized as a novel predictor of adverse events in aortic diseases, but its prognostic value in type A intramural haematoma is unknown. We aimed to evaluate the association between ascending aortic length and outcomes in patients with type A intramural haematoma initially managed medically. We retrospectively analyzed patients with acute type A intramural haematoma. Ascending aortic length was measured by computed tomography. The primary outcome was aortic progression, defined as aortic intervention or aortic-related death. A total of 98 patients were enrolled. During a median follow-up of 2.6 years, aortic progression occurred in 27 patients (27.6%), ie, 9 events per 100 patient-years). Patients with ascending aortic length ≥11 cm had significantly higher rates of aortic progression (54.2% [20.9 events per 100 patient-years] vs 18.9% [6.1 events per 100 patient-years], p = 0.001), surgical intervention (45.8% vs 12.2%, p = 0.001), and presence of ulcer-like projection (25.0% vs 2.7%, p = 0.002) compared to those with ascending aortic length <11 cm. Kaplan-Meier analysis demonstrated lower progression-free survival in the ascending aortic length ≥11 cm group (p = 0.0021). Ascending aortic length had a sensitivity of 61.9% and specificity of 77.8% for predicting aortic progression, with an area under the curve of 0.756 (95% confidence interval: 0.649-0.862). Ascending aortic elongation may identify a high-risk subgroup of acute type A intramural haematoma patients initially managed medically who could potentially benefit from early surgery. Ascending aortic length should be considered in the risk stratification and management of these patients.