BACKGROUND A non-infectious inflammatory reaction against replaced aortic graft for aortic dissection often manifests as fever, malaise, and peri-graft effusion. It usually lasts less than 1 month and subsides spontaneously without immunosuppressive treatment. CASE REPORT A 49-year-old man underwent ascending aorta and total arch replacement for acute thoracic aortic dissection. He had fever, malaise, nausea, and elevated serum C-reactive protein for 1 month postoperatively. Pathological examination of the aorta revealed no aortitis, and repeated blood cultures were negative. We also noted periaortic graft fluid collection, and a small amount of pleural and pericardial effusions. We suspected post-pericardiotomy syndrome. Colchicine and prednisolone were administered, with an excellent clinical response. Three weeks after discontinuation of a 7-week prednisolone treatment, the same symptoms recurred and gradually worsened. Prednisolone was restarted 6 months after the first surgery, with good clinical response. Thereafter, he developed left-sided weakness and dysarthria, being diagnosed as ischemic stroke. Contrast-enhanced computed tomography revealed fluid collection with contrast leak around the aortic grafts, suggesting peel dehiscence, and thrombus formation in anastomotic pseudoaneurysm. He underwent surgical repair. He was diagnosed with non-infectious periaortitis, likely due to an immune reaction to the grafts, based on an excellent clinical response to immunosuppressive therapy. CONCLUSIONS We report a case of non-infectious periaortitis around a thoracic aortic graft, probably with an immune-mediated mechanism, requiring immunosuppressive treatment. When fever persists after aortic graft replacement surgery, non-infectious periaortitis should be considered and immunosuppressive treatment should be considered to prevent critical complications of anastomotic pseudoaneurysm and graft dehiscence.