Our observations raise the question why fibrinogen concentrations are elevated in PAH and CTEPH: First, fibrinogen may be a marker indicating a pro-coagulant condition. Second, fibrinogen may be elevated in response to inflammatory activity in PAH/CTEPH, which could be initiated by a systemic or localized inflammation within the pulmonary vascular bed. Third, fibrinogen per se may be a central mediator of inflammation leading to vascular remodeling in CTEPH.