The patient was a 48-year-old man who had undergone fenestrated frozen elephant trunk (FET) technique for acute type A aortic dissection. Postoperative enhanced computed tomography (CT) imaging revealed a type 1a endoleak from the fenestration. Nevertheless, the stented portion of the FET exhibited circular expansion. However, 2 months after surgery, enhanced CT imaging revealed the flattening of the FET due to the persistent endoleak and we performed an urgent zone 2 thoracic endovascular aortic repair (TEVAR). When type 1a endoleak from the fenestration is observed, the FET can be flattened, as in this patient, additional intervention should be considered.