Pulmonary arterial hypertension (PAH), another main PH group, shares several features indistinguishable from CTEPH: histopathology of peripheral vascular lesions4, acute vasoreactivity testing1, extent of endothelial dysfunction (ED)5 and fibrin resistance to lysis136. However, it is still unknown which exact pathophysiological mechanisms separate these two diseases from each other and in which areas the diseases can be seen as one entity.