We describe the case of a 70-year-old man with Marfan Syndrome who was found to have Diffuse Large B Cell Lymphoma (DLBCL) within an ascending aortic graft, his native aortic arch and arch vessels. Ten years previously he had undergone a Bentall's Procedure for a dilated aortic root. He presented with a left middle cerebral artery stroke. Computed tomography (CT) scan revealed multiple filling defects within the aortic arch and branch vessels suggestive of thrombus. Following the stroke, his neurological deficit improved. A decision was made to perform an aortic arch replacement to reduce the risk of a further embolic event. Subsequent histopathology confirmed DLBCL. Intravascular DLBCL is an extremely rare entity but an important diagnosis to consider in patients with intra-aortic thrombus in proximity to prosthetic graft material.