Pfizer disclosed the death of a participant in a long‑term extension trial of its hemophilia drug Hympavzi (marstacimab). The company said the patient experienced a cerebellar infarction followed by cerebral hemorrhage, and it communicated the case in a letter to the hemophilia community. Pfizer and investigators are reviewing the event to determine causality and any implications for ongoing trials and post‑market safety monitoring. The company did not immediately link the death to treatment but acknowledged heightened scrutiny from clinicians and patient groups. Regulators and trial sites will likely reexamine safety monitoring protocols for trials of hemostasis‑modulating agents where thrombotic and hemorrhagic risks coexist.