In mid-February 2018, a 29-yeal old male patient with Marfan’s syndrome, a history of mitral valve repair in 2007, valve-sparing aortic root replacement in 2017 and aortic valve replacement with a St Jude mechanical heart valve inside the valve sparing aortic root graft in 2018 presented with a sudden onset of palpitations and New York Heart Association Class III symptoms. Computed tomography demonstrated a large, 7 x 4 x 9 cm mediastinal haematoma arising from a long vertical incisional defect in the anterior wall of the Dacron graft. During surgery, hypothermic cardiopulmonary bypass (CPB) was established and after cooling the patient to 16° C, a partial redo sternotomy was performed. During this process, the haematoma was entered, resulting in profuse bleeding. Circulatory arrest was immediately initiated, sternotomy was opened with a chest retractor and mediastinal blood evacuated. A 4.0 x 2.5 cm vertical defect in the Bentall graft with fragments of 4-0 polypropylene sutures were found at its edges. The defect was closed with a Dacron patch, then the graft was de-aired and CPB resumed. After rewarming the patient to 34° C temperature, a direct current shock was applied to convert ventricular fibrillation into sinus rhythm. On fully rewarming the patient, he was uneventfully weaned from CPB and decannulated. In our opinion, long vertical incisional defects in the Bentall Dacron graft should be closed with a synthetic patch. A postoperative mediastinal haematoma after aortic root replacement can be successfully operated on in deep hypothermic circulatory arrest with CPB.