Thirty-five patients with PAH and 22 patients with CTEPH were included in the study after obtaining written informed consent. The study was approved by the local ethics committee. PAH was diagnosed as idiopathic (n = 25) if the evaluation performed before catheterization did not reveal any other causes of elevated pulmonary pressure and was associated with other conditions such as congenital heart disease (n = 6), connective tissue disease (n = 2), and HIV (n = 2) that were diagnosed by medical history, echocardiography, antibody screening, rheumatologic examination, blood analyses, and additional tests if required according to best clinical practice (eg, pulmonary function tests, blood gas assessment, thoracic CT scan, and coronary angiography). CTEPH was diagnosed if both the radioisotope ventilation-perfusion scan showed more than two areas with perfusion defects and pulmonary angiography showed clear evidence of chronic major-vessel thromboembolic disease with disruption or changes in vessel caliber, arterial wall irregularities, transversal bands tethering the arterial lumen, and/or the absence of segmental or lobar arterial branches. None of our patients had medical conditions that would pose a relative contraindication to vasodilator testing due to the possibility of systemic vasodilatation such as that occurring in severe coronary heart or cerebrovascular disease, preexisting systemic hypotension, or other life-threatening illnesses. Patients receiving regular vasodilatative medication were excluded from the study; other medication regimens were kept unchanged. Basic demographics, the 6-min walking distance (6MWD), and the Borg dyspnea scale were assessed according to standard protocols before right heart catheterization and at 3 and 12 months after catheterization; during this time, the patients were treated with either oral or inhaled pulmonary vasodilators.