OBJECTIVE: The outer false lumen wall (FLW) changes from thin/compliant to thick/rigid as aortic dissection transitions from the acute to chronic phase. This study investigates biomechanical stiffness and histological changes of the FLW as the dissected aorta ages. METHODS: The free outer FLW from human tissue was analyzed from chronic type B dissection (chronic-FLW) n = 10, acute type A dissection (acute-FLW) n = 10, and transplant donor descending aorta that was manually peeled into 2 layers (control-FLW) n = 17. Biaxial tension testing in the longitudinal and circumferential directions was performed and stress-strain curves were obtained. A lower and higher tangent modulus was determined to assess stiffness. Quantification of collagen and elastin was performed by calculating the fibers' volume fraction from Z-stack scans. RESULTS: The higher tangent modulus of chronic-FLW is larger ( CONCLUSIONS: As the acutely dissected aorta transitions to the chronic phase, the FL remodels by increasing collagen, decreasing elastin, and increasing aortic stiffness and thickness. This change in the chronic-FLW may be a protective adaptation to prevent FL enlargement and rupture in type B aortic dissection.