We describe the hemodynamic principles that we postulate are responsible for the development of aortic dissection. Two types are differentiated: the "hypertensive" and the "Marfan" types. The former is characterized by moderate aortic dilatation and considerable blood pressure elevation, the latter by considerable aortic dilatation only. The role of wall stress in the development of the intimal tear and the propagation of the process of dissection are discussed. We recommend that patients who are at high risk for developing aortic dissection--those with Marfan's syndrome and dilated aortas--should undergo aortoplasty and wall reinforcement as a simple, safe, and effective way to prevent aortic dissection.