Background Current practice guidelines for patients with thoracic aortic aneurysms (TAAs) recommend 6-12-month intervals for surveillance imaging based on growth estimates of 0.10 - 0.42 cm/year gleaned from limited studies which included patients with thoraco-abdominal aneurysms, known acute or chronic aortic dissection, and other syndromic and non-syndromic high risk conditions (TAA-HRC) associated with high-risk for adverse aortic events and death.   Objective: Our objective was to determine TAA growth and event-free survival rates for patients with aortic root or mid-ascending diameters <5.0 cm, and without thoraco-abdominal aneurysms, acute or chronic aortic dissection or higher risk syndromic or non-syndromic conditions (TAA-NoHRC). Materials and Methods A retrospective review of patient records and imaging studies was done. Aortic diameter measurements were all performed by the lead author. Results For 197 TAA-NoHRC found incidentally during chest imaging, with 616 chest imaging studies over 868 patient-years, the mean aortic root and mid-ascending aortic growth rates were 0.018 and 0.022 cm/year respectively. Growth rate was significantly lower for aneurysms initially measured at <4.5 cm versus > 4.5 cm at both the aortic root (0.011 vs 0.068 cm/year) and mid-ascending aorta (0.013 vs 0.043 cm/year). Survival free from adverse aortic events (dissection, rupture, surgery) or death at 5 years was 99.5%. Conclusions Adult TAA-NoHRC patients with initial aortic root and/or ascending aortic diameters <5.0 cm, and particularly <4.5 cm, have very low aortic growth and adverse event rates which may permit longer intervals between surveillance imaging, up to 3-5 years, after initial (6-12 month) stability is documented.