Despite numerous studies investigating coagulation parameters, we revealed elevated fibrinogen plasma concentrations in CTEPH patients for the first time. Huber et al. found fibrinogen concentrations to be increased in PAH and “secondary” PH as opposed to healthy controls and patients with PH related to Eisenmenger's reaction13. In contrast, Welsh et al. detected increased fibrinogen concentrations only in patients with associated forms of precapillary PH but not in idiopathic PAH14. Interestingly, fibrinogen concentrations correlated with mPAP in the combined population of their study. In our study, we found elevated fibrinogen concentrations in both PAH and CTEPH patients compared to non-PH controls. However, the independent association with haemodynamic parameters (RAP, mPAP) was only found in CTEPH patients. In contrast to all existing studies, our control group was not recruited from healthy individuals but from patients with suspected PH. Despite diseases with known substantial inflammatory activity within the control cohort – i.e. coronary artery disease and systemic sclerosis –, we were able to reveal significantly different fibrinogen concentrations between the control group and PAH or CTEPH patients (regardless of fibrinogen being considered as an acute phase protein).