Here, we describe a totally percutaneous technique for longitudinal fenestration of a chronic dissection flap in the setting of endovascular aneurysm repair (EVAR), where the septum would otherwise preclude proper endograft sealing. This technique is demonstrated in a 65-year-old man with a history of open surgical repair of a Stanford type A aortic dissection, with a type B component that was managed nonoperatively. The patient developed aneurysmal degeneration of the infrarenal aorta during follow-up, and his anatomy was well suited for EVAR with the exception of a chronic dissection flap dividing the proximal seal zone. Using bilateral percutaneous access, a wire was passed through an existing fenestration in the septum from true to false lumen and snared from the contralateral side. Downward traction on this through-wire was then used as a "cheese-wire" to divide the septum longitudinally and clear it from the proximal fixation site. Removal of the septum provided an adequate proximal seal zone for the endograft, and standard infrarenal EVAR was then performed with a good technical result. Longitudinal fenestration using this technique is a useful adjunctive maneuver to facilitate EVAR in the setting of chronic aortic dissection and is safely achievable via a totally percutaneous approach.