Background: In international guidelines, risk estimation for thoracic ascending aortic aneurysm (TAAA) is based on aortic diameter. We previously introduced the aortic size index (ASI), defined as aortic size/body surface area (BSA), as a predictor of aortic dissection, rupture, and death. However, weight might not contribute substantially to aortic size and growth. We seek to evaluate the height-based aortic height index (AHI) versus ASI for risk estimation and revisit our natural history calculations. Methods: Aortic diameters and long-term complications of 780 patients with TAAA were analyzed. Growth rate estimates, yearly complication rates, and survival were assessed. Risk stratification was performed using regression models. The predictive value of AHI and ASI was compared. Results: Patients were stratified into 4 categories of yearly risk of complications based on their ASI and AHI. ASIs (cm/m(2)) of <= 2.05, 2.08 to 2.95, 3.00 to 3.95 and >= 4, and AHIs (cm/m) of <= 2.43, 2.44 to 3.17, 3.21 to 4.06, and >= 4.1 were associated with a 4%, 7%, 12%, and 18% average yearly risk of complications, respectively. Five-year complication-free survival was progressively worse with increasing ASI and AHI. Both ASI and AHI were shown to be significant predictors of complications (P<. 05). AHI categories 3.05 to 3.69, 3.70 to 4.34, and >= 4.35 cm/m were associated with a significantly increased risk of complications (P<. 05). The overall fit of the model using AHI was modestly superior according to the concordance statistic. Conclusions: Compared with indices including weight, the simpler height-based ratio (excluding weight and BSA calculations) yields satisfactory results for evaluating the risk of natural complications in patients with TAAA.