Objective: Management and follow-up of chronic aortic dissections continue to be a clinical challenge due to progressive dilatation and subsequent rupture. To predict complications, guidelines suggest follow-up of aortic diameter. However, dilatation is triggered by hemodynamic parameters (pressures/wall shear stresses) and geometry of false (FL) and true lumen (TL), information not captured by diameter alone. Therefore, we aimed at better understanding the influence of dissection anatomy on TL and FL hemodynamics. Methods: In vitro studies were performed using pulsatile flow in realistic dissected latex/silicone geometries with varying tear number, size, and location. We assessed three different conformations: (1) proximal tear only; (2) distal tear only; (3) both proximal and distal tears. All possible combinations (n = 8) of small (10% of aortic diameter) and large (25% of aortic diameter) tears were considered. Pressure, velocity, and flow patterns were analyzed within the lumina (at proximal and distal sections) and at the tears. We also computed the FL mean pressure index (FPImean%) as a percentage of the TL mean pressure, to compare pressures among models. Results: The presence of large tears equalized FL/TL pressures compared with models with only small tears (proximal FPImean% 99.85 ± 0.45 vs 92.73 ± 3.63; distal FPImean% 99.51 ± 0.80 vs 96.35 ± 1.96; P <.001). Thus, large tears resulted in slower velocities through the tears (systolic velocity <180 cm/s) and complex flows within the FL, whereas small tears resulted in lower FL pressures, higher tear velocities (systolic velocity >290 cm/s), and a well-defined flow. Additionally, both proximal and distal tears act as entry and exit. During systole, flow enters the FL through all tears simultaneously, while during diastole, flow leaves through all communications. Flow through the FL, from proximal to distal tears or vice versa, is minimal. Conclusions: Our results suggest that FL hemodynamics heavily depends on cumulative tear size, and thus, it is an important parameter to take into account when clinically assessing chronic aortic dissections. © 2013 Society for Vascular Surgery.