Pfizer disclosed the death of a participant in a long‑term extension trial of Hympavzi (marstacimab), reporting a cerebellar infarction followed by cerebral hemorrhage. The company notified the hemophilia community and regulators; the event has drawn attention to safety monitoring in trials of tissue factor pathway inhibitor antagonists. Pfizer and external investigators are reviewing the case. The disclosure could affect ongoing enrollment and the benefit‑risk assessment for Hympavzi as data continue to accumulate in extension cohorts.