Reports of thrombotic complications in patients with COVID-19 are increasingly prominent, and these reports include patients receiving therapeutic anticoagulation.1, 2 At our institution, multiple occurrences of anticoagulation failure prompted us to search for alternative aetiologies contributing to refractory hypercoagulability. Here we describe COVID-19-associated hyperviscosity, a potentially severe consequence of infection with severe acute respiratory syndrome coronavirus 2, in 15 patients tested to date. This work was done ethically in accordance with institutional review board approval.