In summary, fibrinogen plasma concentrations are elevated in PAH and CTEPH patients compared to control patients at time of initial RHC. In CTEPH but not in PAH patients increased fibrinogen concentrations predict impaired haemodynamic parameters and clinical severity. This correlation, which is supported by experimental data from other groups in PA-EC and PA-SMC signalling, together with the fact that established risk factors for CTEPH seem to be associated with elevated fibrinogen concentrations, indicate a central role of fibrinogen in the pathophysiology of CTEPH. Therefore, we strongly believe that fibrinogen is a hitherto unrecognized diagnostic and disease severity marker in CTEPH, which warrants further experimental and clinical studies.