Growth rate: the aneurysmal thoracic aorta grows at an average rate of cm per year ( for ascending and for descending). Critical sizes: hinge points for natural complications of aortic aneurysm (rupture or dissection) were found at cm for the ascending aorta and cm for the descending. By the time a patient achieved these critical dimensions the likelihood of rupture or dissection was 31% for the ascending and 43% for the descending aorta. Yearly event rates: a patient with an aorta that has reached 6 cm maximal diameter faces the following yearly rates of devastating adverse events: rupture (%), dissection (%), death (%), rupture, dissection, or death (%). Surgical risks: risk of death from aortic surgery for thoracic aortic aneurysm was % for the ascending and arch and 8% for the descending and thoracoabdominal aorta. Genetic analysis: family pedigrees confirm that 21% of probands with thoracic aortic aneurysm have first-order family members with arterial aneurysm.