We report hypercoagulability despite activated partial thromboplastin time (APTT)-guided heparin treatment during Berlin Heart-supported circulation in a 38-year-old man with heart failure for 19 days. The patient was anticoagulated using unfractionated heparin, acetylsalicylic acid and dipyridamole. Contact and tissue factor-activated thromboelastometry revealed increased clot firmness, although anticoagulation assessed by APTT was in accordance with the treatment protocol. Strength of polymerized fibrin was also increased. We saw no clinical signs of thrombosis. Thromboelastometry normalized after heart transplantation. Our results suggest that hypercoagulability is due to excess fibrin formation. Monitoring anticoagulation using APTT may, therefore, be misleading during mechanical cardiac assist.